Trigonometric Identities and Equations

Transcription

1 Trigonometric Identities and Equations Art Fortgang, (ArtF) Lori Jordan, (LoriJ) Say Thanks to the Authors Click (No sign in required)

2 To access a customizable version of this book, as well as other interactive content, visit AUTHORS Art Fortgang, (ArtF) Lori Jordan, (LoriJ) CK- Foundation is a non-profit organization with a mission to reduce the cost of textbook materials for the K- market both in the U.S. and worldwide. Using an open-content, web-based collaborative model termed the FlexBook, CK- intends to pioneer the generation and distribution of high-quality educational content that will serve both as core text as well as provide an adaptive environment for learning, powered through the FlexBook Platform. Copyright 0 CK- Foundation, The names CK- and CK and associated logos and the terms FlexBook and FlexBook Platform (collectively CK- Marks ) are trademarks and service marks of CK- Foundation and are protected by federal, state, and international laws. Any form of reproduction of this book in any format or medium, in whole or in sections must include the referral attribution link (placed in a visible location) in addition to the following terms. Except as otherwise noted, all CK- Content (including CK- Curriculum Material) is made available to Users in accordance with the Creative Commons Attribution/Non- Commercial/Share Alike.0 Unported (CC BY-NC-SA) License ( as amended and updated by Creative Commons from time to time (the CC License ), which is incorporated herein by this reference. Complete terms can be found at Printed: August, 0

3 Chapter. Trigonometric Identities and Equations CHAPTER Trigonometric Identities and Equations CHAPTER OUTLINE. Fundamental Identities. Proving Identities. Solving Trigonometric Equations.4 Sum and Difference Identities.5 Double Angle Identities. Half-Angle Identities.7 Products, Sums, Linear Combinations, and Applications

4 .. Fundamental Identities Fundamental Identities Introduction We now enter into the proof portion of trigonometry. Starting with the basic definitions of sine, cosine, and tangent, identities (or fundamental trigonometric equations) emerge. Students will learn how to prove certain identities, using other identities and definitions. Finally, students will be able solve trigonometric equations for theta, also using identities and definitions. Learning Objectives use the fundamental identities to prove other identities. apply the fundamental identities to values of θ and show that they are true. Quotient Identity In Chapter, the three fundamental trigonometric functions sine, cosine and tangent were introduced. All three functions can be defined in terms of a right triangle or the unit circle.

5 Chapter. Trigonometric Identities and Equations sinθ opposite hypotenuse y r y y ad jacent cosθ hypotenuse x r x x tanθ opposite ad jacent y x sinθ cosθ The Quotient Identity is tanθ sinθ cosθ. We see that this is true because tangent is equal to y x, in the unit circle. We know that y is equal to the sine values of θ and x is equal to the cosine values of θ. Substituting sinθ for y and cosθ for x and we have a new identity. Example : Use θ 45 to show that tanθ sinθ cosθ holds true. Solution: Plugging in 45, we have: tan45 sin45 cos45. Then, substitute each function with its actual value and simplify both sides. sin45 cos45 and we know that tan 45, so this is true. Example : Show that tan 90 is undefined using the Quotient Identity. Solution: tan90 sin90 cos90 0, because you cannot divide by zero, the tangent at 90 is undefined. Reciprocal Identities Chapter also introduced us to the idea that the three fundamental reciprocal trigonometric functions are cosecant (csc), secant (sec) and cotangent (cot) and are defined as: cscθ sinθ secθ cosθ cotθ tanθ If we apply the Quotient Identity to the reciprocal of tangent, an additional quotient is created: Example : Prove tanθ sinθsecθ cotθ tanθ sinθ cosθ cosθ sinθ Solution: First, you should change everything into sine and cosine. Feel free to work from either side, as long as the end result from both sides ends up being the same. tanθ sinθ secθ sinθ sinθ cosθ cosθ Here, we end up with the Quotient Identity, which we know is true. Therefore, this identity is also true and we are finished.

6 .. Fundamental Identities Example 4: Given sinθ 5 and θ is in the fourth quadrant, find secθ. Solution: Secant is the reciprocal of cosine, so we need to find the adjacent side. We are given the opposite side, and the hypotenuse, 5. Because θ is in the fourth quadrant, cosine will be positive. From the Pythagorean Theorem, the third side is: ( ) + b 5 + b 5 b 9 b 9 From this we can now find cosθ 9 5. Since secant is the reciprocal of cosine, secθ 5 9, or Pythagorean Identity Using the fundamental trig functions, sine and cosine and some basic algebra can reveal some interesting trigonometric relationships. Note when a trig function such as sin θ is multiplied by itself, the mathematical convention is to write it as sin θ. (sinθ can be interpreted as the sine of the square of the angle, and is therefore avoided.) sin θ y and cos θ x or sin θ + cos θ y + x x +y r r r r r r Using the Pythagorean Theorem for the triangle above: x + y r Then, divide both sides by r, x +y r r. So, because x +y,sin θ + cos θ also equals. This is known as r r the Trigonometric Pythagorean Theorem or the Pythagorean Identity and is written sin θ + cos θ. Alternative forms of the Theorem are: +cot θ csc θ and tan θ+ sec θ. The second form is found by taking the original form and dividing each of the terms by sin θ, while the third form is found by dividing all the terms of the first by cos θ. Example 5: Use 0 to show that sin θ + cos θ holds true. Solution: Plug in 0 and find the values of sin0 and cos0. sin 0 + cos 0 ( ) ( ) Even and Odd Identities Functions are even or odd depending on how the end behavior of the graphical representation looks. For example, y x is considered an even function because the ends of the parabola both point in the same direction and the parabola is symmetric about the y axis. y x is considered an odd function for the opposite reason. The ends of 4

7 Chapter. Trigonometric Identities and Equations a cubic function point in opposite directions and therefore the parabola is not symmetric about the y axis. What about the trig functions? They do not have exponents to give us the even or odd clue (when the degree is even, a function is even, when the degree is odd, a function is odd). Even Function Odd Function y ( x) x y ( x) x Let s consider sine. Start with sin( x). Will it equal sinx or sinx? Plug in a couple of values to see. sin( 0 ) sin0 sin0 sin( 5 ) sin5 sin5 From this we see that sine is odd. Therefore, sin( x) sinx, for any value of x. For cosine, we will plug in a couple of values to determine if it s even or odd. cos( 0 ) cos0 cos0 cos( 5 ) cos5 cos5 This tells us that the cosine is even. Therefore, cos( x) cosx, for any value of x. The other four trigonometric functions are as follows: tan( x) tanx csc( x) cscx sec( x) secx cot( x) cotx Notice that cosecant is odd like sine and secant is even like cosine. Example : If cos( x) 4 and tan( x) 7, find sinx. Solution: We know that sine is odd. Cosine is even, so cosx 4. Tangent is odd, so tanx 7. Therefore, sine is positive and sinx 7 4. Cofunction Identities Recall that two angles are complementary if their sum is 90. In every triangle, the sum of the interior angles is 80 and the right angle has a measure of 90. Therefore, the two remaining acute angles of the triangle have a sum equal to 90, and are complementary. Let s explore this concept to identify the relationship between a function of one angle and the function of its complement in any right triangle, or the cofunction identities. A cofunction is a pair of trigonometric functions that are equal when the variable in one function is the complement in the other. In ABC, C is a right angle, A and B are complementary. 5

8 .. Fundamental Identities Chapter introduced the cofunction identities (section.8) and because A and B are complementary, it was found that sina cosb,cosa sinb,tana cotb,cota tanb,csca secb and seca cscb. For each of the above A π B. To generalize, sin ( π θ) cosθ and cos ( π θ) sinθ,tan ( π θ) cotθ and cot ( π θ) tanθ,csc ( π θ) secθ and sec ( π θ) cscθ. The following graph represents two complete cycles of y sinx and y cosθ. Notice that a phase shift of π on y cosx, would make these graphs exactly the same. These cofunction identities hold true for all real numbers for which both sides of the equation are defined. Example 7: Use the cofunction identities to evaluate each of the following expressions: a. If tan ( π θ) 4. determine cotθ b. If sinθ 0.9 determine cos ( π θ). Solution: a. tan ( π θ) cotθ therefore cotθ 4. b. cos ( π θ) sinθ therefore cos ( π θ) 0.9 Example 8: Show sin ( π x) cos( x) is true. Solution: Using the identities we have derived in this section, sin ( π x) cosx, and we know that cosine is an even function so cos( x) cosx. Therefore, each side is equal to cosx and thus equal to each other. Points to Consider Why do you think secant is even like cosine? How could you show that tangent is odd?

9 Chapter. Trigonometric Identities and Equations Review Questions. Use the Quotient Identity to show that the tan 70 is undefined.. If cos ( π x) 4 5, find sin( x).. If tan( x) 5 5 and sinx, find cosx. 4. Simplify secxcos ( π x). 5. Verify sin θ + cos θ using: a. the sides 5,, and of a right triangle, in the first quadrant b. the ratios from a triangle. Prove + tan θ sec θ using the Pythagorean Identity 7. If cscz and cosz 7, find cotz. 8. Factor: a. sin θ cos θ b. sin θ + sinθ Simplify sin4 θ cos 4 θ using the trig identities sin θ cos θ 0. Rewrite cosx secx so that it is only in terms of cosine. Simplify completely.. Prove that tangent is an odd function. Review Answers. tan70 sin70 0, you cannot divide by zero, therefore tan70 is undefined.. If cos ( π x) 4 5, then, by the cofunction identities, sinx 4 5. Because sine is odd, sin( x) If tan( x) 5, then tanx 5. Because sinx 5, cosine is also negative, so cosx. 4. Use the reciprocal and cofunction identities to simplify ( π ) secxcos x cos70 cosx sinx sinx cosx tanx 5. (a) Using the sides 5,, and and in the first quadrant, it doesn t really matter which is cosine or sine. So, sin θ + cos θ becomes ( ) 5 ( + ). Simplifying, we get: , and finally 9. (b) sin θ + cos θ becomes ( ( ) ) +. Simplifying we get: and To prove tan θ + sec θ, first use sinθ cosθ tanθ and change sec θ cos θ. 7. If cscz 7 8 tan θ + sec θ sin θ cos θ + cos θ sin θ cos θ + cos θ cos θ cos θ sin θ + cos θ 5 8 and cosz 7, then sinz 7 and tanz Therefore cotz 8. 7

10 .. Fundamental Identities 8. (a) Factor sin θ cos θ using the difference of squares. sin θ cos θ (sinθ + cosθ)(sinθ cosθ) (b) sin θ + sinθ + 8 (sinθ + 4)(sinθ + ) 9. You will need to factor and use the sin θ + cos θ identity. sin 4 θ cos 4 θ sin θ cos θ (sin θ cos θ)(sin θ + cos θ) sin θ cos θ sin θ + cos θ 0. To rewrite cosx secx so it is only in terms of cosine, start with changing secant to cosine. cosx secx cosx cosx cosx cosx cosx cosx cosx Now, simplify the denominator. cosx Multiply by the reciprocal cosx cosx cosx cosx cosx cosx cosx cos x cosx cosx. The easiest way to prove that tangent is odd to break it down, using the Quotient Identity. tan( x) sin( x) cos( x) sinx cosx tanx from this statement, we need to show that tan( x) tanx because sin( x) sinx and cos( x) cosx 8

11 Chapter. Trigonometric Identities and Equations. Proving Identities Learning Objectives Prove identities using several techniques. Working with Trigonometric Identities During the course, you will see complex trigonometric expressions. Often, complex trigonometric expressions can be equivalent to less complex expressions. The process for showing two trigonometric expressions to be equivalent (regardless of the value of the angle) is known as validating or proving trigonometric identities. There are several options a student can use when proving a trigonometric identity. Option One: Often one of the steps for proving identities is to change each term into their sine and cosine equivalents: Example : Prove the identity: cscθ tanθ secθ Solution: Reducing each side separately. It might be helpful to put a line down, through the equals sign. Because we are proving this identity, we don t know if the two sides are equal, so wait until the end to include the equality. cscx tanx sinx sinx cosx sinx sinx cosx cosx secx cosx cosx cosx At the end we ended up with the same thing, so we know that this is a valid identity. Notice when working with identities, unlike equations, conversions and mathematical operations are performed only on one side of the identity. In more complex identities sometimes both sides of the identity are simplified or expanded. The thought process for establishing identities is to view each side of the identity separately, and at the end to show that both sides do in fact transform into identical mathematical statements. Option Two: Use the Trigonometric Pythagorean Theorem and other Fundamental Identities. Example : Prove the identity: ( cos x)( + cot x) Solution: Use the Pythagorean Identity and its alternate form. Manipulate sin θ + cos θ to be sin θ cos θ. Also substitute csc x for + cot x, then cross-cancel. ( cos x)( + cot x) sin x csc x sin x sin x 9

12 .. Proving Identities Option Three: When working with identities where there are fractions- combine using algebraic techniques for adding expressions with unlike denominators: Example : Prove the identity: sinθ +cosθ + +cosθ sinθ cscθ. Solution: Combine the two fractions on the left side of the equation by finding the common denominator: ( + cosθ) sinθ, and the change the right side into terms of sine. sinθ +cosθ + +cosθ sinθ sinθ sinθ sinθ +cosθ + +cosθ sinθ +cosθ +cosθ sin θ+(+cosθ) sinθ(+cosθ) cscθ csc θ csc θ Now, we need to apply another algebraic technique, FOIL. (FOIL is a memory device that describes the process for multiplying two binomials, meaning multiplying the First two terms, the Outer two terms, the Inner two terms, and then the Last two terms, and then summing the four products.) Always leave the denominator factored, because you might be able to cancel something out at the end. sin θ++cosθ+cos θ sinθ(+cosθ) csc θ Using the second option, substitute sin θ + cos θ and simplify. ++cosθ sinθ(+cosθ) +cosθ sinθ(+cosθ) (+cosθ) sinθ(+cosθ) sinθ csc θ csc θ csc θ sinθ Option Four: If possible, factor trigonometric expressions. Actually procedure four was used in the above example: cscθ can be factored to (+cosθ) cscθ and in this situation, the factors cancel each other. +cosθ sinθ(+cosθ) sinθ(+cosθ) Example 4: Prove the identity: +tanθ (+cotθ) tanθ. Solution: Change cotθ to tanθ and find a common denominator. + tanθ ( ) tanθ + tanθ + tanθ ( tanθ tanθ + ) tanθ or tanθ + tanθ tanθ+ tanθ tanθ Now invert the denominator and multiply. tanθ( + tanθ) tanθ tanθ + tanθ tanθ 0

13 Chapter. Trigonometric Identities and Equations Technology Note A graphing calculator can help provide the correctness of an identity. For example looking at: cscx tanx secx, first graph y cscx tanx, and then graph y secx. Examining the viewing screen for each demonstrates that the results produce the same graph. To summarize, when verifying a trigonometric identity, use the following tips:. Work on one side of the identity- usually the more complicated looking side.. Try rewriting all given expressions in terms of sine and cosine.. If there are fractions involved, combine them. 4. After combining fractions, if the resulting fraction can be reduced, reduce it. 5. The goal is to make one side look exactly like the other so as you change one side of the identity, look at the other side for a potential hint to what to do next. If you are stumped, work with the other side. Don t limit yourself to working only on the left side, a problem might require you to work on the right. Points to Consider Are there other techniques that you could use to prove identities? What else, besides what is listed in this section, do you think would be useful in proving identities? Review Questions Prove the following identities true:

14 .. Proving Identities sinxtanx + cosx secx. cosx cosxsin x cos x sinx. +cosx + +cosx sinx cscx sinx 4. +cosx cosx sinx 5. +cosa + cosa + cot a. cos 4 b sin 4 b sin b 7. secycscy siny+cosy siny cosy siny cosy 8. (secx tanx) sinx +sinx 9. Show that sinxcosx sinx is true using 5π. 0. Use the trig identities to prove sec x cot x csc x Review Answers. Step : Change everything into sine and cosine Step : Give everything a common denominator, cosx. sinxtanx + cosx secx sinx sinx cosx + cosx cosx sin x cosx + cos x cosx cosx Step : Because the denominators are all the same, we can eliminate them. sin x + cos x We know this is true because it is the Trig Pythagorean Theorem. Step : Pull out a cosx cosx cosxsin x cos x cosx( sin x) cos x Step : We know sin x + cos x, so cos x sin x is also true, therefore cosx(cos x) cos x. This, of course, is true, we are done!. Step : Change everything in to sine and cosine and find a common denominator for left hand side. sinx + cosx + + cosx cscx sinx sinx + cosx + + cosx LCD : sinx( + cosx) sinx sinx sin x + ( + cosx) sinx( + cosx)

15 Chapter. Trigonometric Identities and Equations Step : Working with the left side, FOIL and simplify. 4. Step : Cross-multiply sin x + + cosx + cos x sinx( + cosx) sin x + cos x + + cosx sinx( + cosx) + + cosx sinx( + cosx) + cosx sinx( + cosx) ( + cosx) sinx( + cosx) sinx FOIL ( + cosx) move cos x sin x + cos x add fator out cancel ( + cosx) sinx + cosx cosx sinx sin x ( + cosx)( cosx) Step : Factor and simplify sin x cos x sin x + cos x 5. Step : Work with left hand side, find common denominator, FOIL and simplify, using sin x + cos x. + cosx + cosx + cot x cosx + + cosx ( + cosx)( cosx) cos x sin x Step : Work with the right hand side, to hopefully end up with sin x. + cot x + cos x ( sin x ) + cos x sin x ( sin x + cos ) x sin x ( ) sin x sin x Both sides match up, the identity is true. factor out the common denominator trig pythagorean theorem simply/multiply

16 .. Proving Identities Step : Factor left hand side cos 4 b sin 4 b (cos b + sin b)(cos b sin b) cos b sin b sin b sin b sin b Step : Substitute sin b for cos b because sin x + cos x. ( sin b) sin b sin b sin b sin b sin b sin b sin b 7. Step : Find a common denominator for the left hand side and change right side in terms of sine and cosine. siny + cosy cosy siny secycscy siny cosy cosy(siny + cosy) siny(cosy siny) sinycosy sinycosy Step : Work with left side, simplify and distribute. sinycosy + cos y sinycosy + sin y sinycosy cos y + sin y sinycosy sinycosy 8. Step : Work with left side, change everything into terms of sine and cosine. (secx tanx) sinx + sinx ( cosx sinx ) cosx ( ) sinx cosx ( sinx) cos x Step : Substitute sin x for cos x because sin x + cos x ( sinx) sin be careful, these are NOT the same! x Step : Factor the denominator and cancel out like terms. ( sinx) ( + sinx)( sinx) sinx + sinx 9. Plug in 5π for x into the formula and simplify. sinxcosx sinx sin 5π cos 5π sin 5π ( ) ( ) sin 5π 4

17 Chapter. Trigonometric Identities and Equations This is true because sin00 is 0. Change everything into terms of sine and cosine and simplify. secxcotx cscx cosx cosx sinx sinx sinx sinx 5

18 .. Solving Trigonometric Equations Solving Trigonometric Equations Learning Objectives Use the fundamental identities to solve trigonometric equations. Express trigonometric expressions in simplest form. Solve trigonometric equations by factoring. Solve trigonometric equations by using the Quadratic Formula. By now we have seen trigonometric functions represented in many ways: Ratios between the side lengths of right triangles, as functions of coordinates as one travels along the unit circle and as abstract functions with graphs. Now it is time to make use of the properties of the trigonometric functions to gain knowledge of the connections between the functions themselves. The patterns of these connections can be applied to simplify trigonometric expressions and to solve trigonometric equations. Simplifying Trigonometric Expressions Example : Simplify the following expressions using the basic trigonometric identities: a. +tan x csc x b. sin x+tan x+cos x secx c. cosx cos x Solution: a. + tan x csc...( + tan x sec x)pythagorean Identity x sec x csc x...(sec x cos x and csc x sin )Reciprocal Identity x ( ) ( ) cos x cos x sin x sin x ( ) ( sin ) x cos sin x x cos x tan x Quotient Identity b.

19 Chapter. Trigonometric Identities and Equations c. sin x + tan x + cos x...(sin x + cos x )Pythagorean Identity secx + tan x...( + tan x sec x)pythagorean Identity secx sec x secx secx cosx cos x cosx( cos x) cosx(sin x)...factor out cosx and sin x cos x In the above examples, the given expressions were simplified by applying the patterns of the basic trigonometric identities. We can also apply the fundamental identities to trigonometric equations to solve for x. When solving trig equations, restrictions on x (or θ) must be provided, or else there would be infinitely many possible answers (because of the periodicity of trig functions). Solving Trigonometric Equations Example : Without the use of technology, find all solutions tan (x), such that 0 x π. Solution: tan x tan x tanx ± This means that there are four answers for x, because tangent is positive in the first and third quadrants and negative in the second and fourth. Combine that with the values that we know would generate tanx or tanx,x π, π, 4π, and 5π. Example : Solve cos x sin x cos x 0 for all values of x between [0, π]. Solution: cosx (sinx ) 0 set each factor equal to zero and solve them separately cosx 0 sinx x π and x π sinx x π and x 5π 7

20 .. Solving Trigonometric Equations In the above examples, exact values were obtained for the solutions of the equations. These solutions were within the domain that was specified. Example 4: Solve sin x cosx 0 for all values of x. Solution: The equation now has two functions sine and cosine. Study the equation carefully and decide in which function to rewrite the equation. sin x can be expressed in terms of cosine by manipulating the Pythagorean Identity, sin x + cos x. sin x cosx 0 ( cos x) cosx 0 cos x cosx 0 cos x cosx + 0 cos x + cosx 0 (cosx )(cosx + ) 0 cosx 0 or cosx + 0 cosx x π + πk,kεz cosx x π + πk,kεz x 5π + πk,kεz Solving Trigonometric Equations Using Factoring Algebraic skills like factoring and substitution that are used to solve various equations are very useful when solving trigonometric equations. As with algebraic expressions, one must be careful to avoid dividing by zero during these maneuvers. Example 5: Solve sin x sinx + 0 for 0 < x π. Solution: sin x sinx + 0 Factor this like a quadratic equation (sinx )(sinx ) 0 sinx 0 or sinx 0 sinx sinx sinx x π x π and x 5π Example : Solve tanxsinx + sinx tanx + for all values of x. Solution: 8

21 Chapter. Trigonometric Identities and Equations Pull out sinx There is a common factor of (tanx + ) Think of the (tanx + ) as ( )(tanx + ), which is why there is a behind the sinx. Example 7: Solve sin x + sinx 0 for all x,[0,π]. Solution: sin x + sinx 0 Factor like a quadratic (sinx )(sinx + ) 0 sinx 0 sinx + 0 sinx x π and x 5π sinx There is no solution because the range of sinx is [,]. Some trigonometric equations have no solutions. This means that there is no replacement for the variable that will result in a true expression. Example 8: Solve 4sin x + sin x sinx 0 for x in the interval [0,π]. Solution: Even though this does not look like a factoring problem, it is. We are going to use factoring by grouping, from Algebra II. First group together the first two terms and the last two terms. Then find the greatest common factor for each pair. 4sin x + sin x } {{ } sinx } {{ } 0 sin x(sinx + ) (sinx + ) Notice we have gone from four terms to two. These new two terms have a common factor of sinx +. We can pull this common factor out and reduce our number of terms from two to one, comprised of two factors. sin x(sinx + ) (sinx + ) 0 (sinx + )(sin x ) 0 9

22 .. Solving Trigonometric Equations We can take this one step further because sin x can factor again. ( )( ) (sinx + ) sinx sinx + 0 Set each factor equal to zero and solve. sinx + 0 sinx sinx x 7π, π or sinx + 0 or sinx 0 sinx sinx sinx sinx x 5π 4, 7π x π 4 4, π 4 Notice there are six solutions for x. Graphing the original function would show that the equation crosses the x axis six times in the interval [0,π]. Solving Trigonometric Equations Using the Quadratic Formula When solving quadratic equations that do not factor, the quadratic formula is often used. The same can be applied when solving trigonometric equations that do not factor. The values for a is the numerical coefficient of the function s squared term, b is the numerical coefficient of the function term that is to the first power and c is a constant. The formula will result in two answers and both will have to be evaluated within the designated interval. Example 9: Solve cot x cotx for exact values of x over the interval [0,π]. Solution: cot x cotx cot x cotx 0 The equation will not factor. Use the quadratic formula for cotx, a,b,c. 0

23 Chapter. Trigonometric Identities and Equations cotx b ± b 4ac a cotx ( ) ± ( ) 4()( ) () cotx ± 9 + cotx + cotx or cotx cotx 4.58 cotx.8 cotx 0.8 tanx tanx x 0.94,.8099 x.887, Example 0: Solve 5cos x + 9sinx + 0 for values of x over the interval [0,π]. Solution: Change cos x to sin x from the Pythagorean Identity. 5cos x + 9sinx + 0 5( sin x) + 9sinx sin x + 9sinx + 0 5sin x + 9sinx 0 x.0 rad and π.0.94 sinx 9 ± 9 4(5)( ) (5) sinx 9 ± sinx 9 ± sinx and sinx 0 sinx 5 and sin (0.) and sin ( ) This is the only solutions for x since is not in the range of values. 9 0 To summarize, to solve a trigonometric equation, you can use the following techniques:. Simplify expressions with the fundamental identities.. Factor, pull out common factors, use factoring by grouping.. The Quadratic Formula. 4. Be aware of the intervals for x. Make sure your final answer is in the specified domain.

24 .. Solving Trigonometric Equations Points to Consider Are there other methods for solving equations that can be adapted to solving trigonometric equations? Will any of the trigonometric equations involve solving quadratic equations? Is there a way to solve a trigonometric equation that will not factor? Is substitution of a function with an identity a feasible approach to solving a trigonometric equation? Review Questions. Solve the equation sinθ 0. for 0 θ < π.. Solve the equation cos x over the interval [0,π]. Solve the trigonometric equation tan x for all values of θ such that 0 θ π 4. Solve the trigonometric equation 4sinxcosx + cosx sinx 0 such that 0 x < π. 5. Solve sin x sinx 0 for x over [0,π].. Solve tan x tanx for x over [0,π]. 7. Find all the solutions for the trigonometric equation sin x 4 cos x 4 0 over the interval [0,π). 8. Solve the trigonometric equation sin x 8sinx over the interval [0,π]. 9. Solve sinxtanx tanx + secx for all values of x ε[0,π]. 0. Solve the trigonometric equation cos x + sinx 0 over the interval [0,π].. Solve tan x + tanx 0 for values of x over the interval [ π, π ].. Solve the trigonometric equation such that 5cos θ sinθ 0 over the interval [0,π]. Review Answers. Because the problem deals with θ, the domain values must be doubled, making the domain 0 θ < 4π The reference angle is α sin θ 0.45,π 0.45,π ,π 0.45θ 0.45,.4980,.9,8.78 The values for θ are needed so the above values must be divided by. θ 0.8,.490,.4, 4.90 The results can readily be checked by graphing the function. The four results are reasonable since they are the only results indicated on the graph that satisfy sinθ 0..

25 Chapter. Trigonometric Identities and Equations. cos x cos x cosx ± 4 Then cosx 4 cos 4 x x.8 radians or cosx 4 cos 4 x x.85 radians However, cosx is also positive in the fourth quadrant, so the other possible solution for cosx 4 is π radians and cosx is also negative in the third quadrant, so the other possible solution for cosx 4 is π radians. tan x tanx ± tanx ± so, tanx or tanx. Therefore, x is all critical values corresponding with π 4 π 4, π 4, 5π 4, 7π 4 4. Use factoring by grouping. within the interval. x sinx + 0 or cosx 0 sinx cosx 5. You can factor this one like a quadratic. sinx x 7π, π cosx x π, 5π sin x sinx 0 (sinx )(sinx + ) 0 sinx 0 sinx + 0 sinx or sinx x sin () x π For this problem the only solution is π because sine cannot be (it is not in the range).

26 .. Solving Trigonometric Equations tan x tanx tan x tanx 0 tanx(tanx ) 0 tanx 0 or tanx x 0,π x.5 7. sin x 4 cos x 4 0 ( cos x ) cos x cos x 4 cos x 4 0 cos x 4 + cos x 4 0 (cos x 4 )(cos x 4 + ) 0 cos x 4 0 or cos x cos x 4 cos x 4 cos x 4 x 4 π 5π or x 4π or 0π 8. 0π is eliminated as a solution because it is outside of the range and cos x 4 will not generate any solutions because is outside of the range of cosine. Therefore, the only solution is 4π. sin x 8sinx sin x 8sinx 0 sin x + 8sinx 0 (sinx )(sinx + ) 0 sinx 0 or sinx + 0 sinx sinx sinx x 0.98 radians No solution exists x π radians 4

27 Chapter. Trigonometric Identities and Equations 9. sinxtanx tanx + secx sinx sinx cosx sinx cosx + cosx sin x cosx sinx + cosx sin x sinx + sin x sinx 0 (sinx + )(sinx ) 0 sinx + 0 or sinx 0 sinx sinx sinx x 7π, π 0. One of the solutions is not π, because tanx and secx in the original equation are undefined for this value of x. cos x + sinx 0 ( sin x) + sinx 0 Pythagorean Identity sin x + sinx 0 sin x + sinx 0 Multiply by sin x sinx + 0 (sinx )(sinx ) 0 sinx 0 or sinx 0 sinx sinx sinx x π, 5π x π. tan x + tanx 0 ± 4()( ) tanx ± + 8 tanx ± tanx tanx or tanx when x π 4, in the interval [ π, π ] tanx when x.07 rad 5

28 .. Solving Trigonometric Equations 5cos θ sinθ 0 over the interval [0,π]. x sin ( ) ( or sin 5 ( sin x ) sinx 0 5sin x sinx sin x + sinx 5 0 ± 4(5)( 5) (5) sinx ± + 00 sinx 0 ± sinx 0 ± 4 sinx 0 ± 4 sinx 5 ) 4 5 x 0.08 rad or.598 rad from the first expression, the second expression will not yield any answers because it is out the the range of sine.

29 Chapter. Trigonometric Identities and Equations.4 Sum and Difference Identities Learning Objectives Use and identify the sum and difference identities. Apply the sum and difference identities to solve trigonometric equations. Find the exact value of a trigonometric function for certain angles. In this section we are going to explore cos(a ± b),sin(a ± b), and tan(a ± b). These identities have very useful expansions and can help to solve identities and equations. Sum and Difference Formulas: Cosine Is cos5 cos(45 0 )? Upon appearance, yes, it is. This section explores how to find an expression that would equal cos(45 0 ). To simplify this, let the two given angles be a and b where 0 < b < a < π. Begin with the unit circle and place the angles a and b in standard position as shown in Figure A. Point Pt lies on the terminal side of b, so its coordinates are (cosb,sinb) and Point Pt lies on the terminal side of a so its coordinates are (cosa,sina). Place the a b in standard position, as shown in Figure B. The point A has coordinates (,0) and the Pt is on the terminal side of the angle a b, so its coordinates are (cos[a b],sin[a b]). 7

30 .4. Sum and Difference Identities Triangles OP P in figure A and Triangle OAP in figure B are congruent. (Two sides and the included angle, a b, are equal). Therefore the unknown side of each triangle must also be equal. That is: d (A,P ) d (P,P ) Applying the distance formula to the triangles in Figures A and B and setting them equal to each other: [cos(a b) ] + [sin(a b) 0] (cosa cosb) + (sina sinb) Square both sides to eliminate the square root. [cos(a b) ] + [sin(a b) 0] (cosa cosb) + (sina sinb) FOIL all four squared expressions and simplify. cos (a b) cos(a b) + + sin (a b) cos a cosacosb + cos b + sin a sinasinb + sin b sin (a b) + cos (a b) } {{ } cos(a b) + } sin a + {{ cos a } cosacosb + } sin b + {{ cos b } sinasinb cos(a b) + cosacosb + sinasinb cos(a b) cosacosb sinasinb cos(a b) cosacosb sinasinb cos(a b) cosacosb + sinasinb In cos(a b) cosacosb + sinasinb, the difference formula for cosine, you can substitute a ( b) a + b to obtain: cos(a + b) cos[a ( b)] or cosacos( b) + sinasin( b). since cos( b) cosb and sin( b) sinb, then cos(a + b) cos a cos b sin a sin b, which is the sum formula for cosine. Using the Sum and Difference Identities of Cosine The sum/difference formulas for cosine can be used to establish other identities: 8

31 Chapter. Trigonometric Identities and Equations Example : Find an equivalent form of cos ( π θ) using the cosine difference formula. Solution: ( π ) cos θ cos π cosθ + sin π sinθ ( π ) cos θ 0 cosθ + sinθ, substitute cos π 0 and sin π ( π ) cos θ sinθ We know that is a true identity because of our understanding of the sine and cosine curves, which are a phase shift of π off from each other. The cosine formulas can also be used to find exact values of cosine that we weren t able to find before, such as 5 (45 0 ),75 ( ), among others. Example : Find the exact value of cos5 Solution: Use the difference formula where a 45 and b 0. Example : Find the exact value of cos05. cos(45 0 ) cos45 cos0 + sin45 sin0 cos5 + + cos5 4 Solution: There may be more than one pair of key angles that can add up (or subtract to) 05. Both pairs, and 50 45, will yield the correct answer... cos05 cos( ) cos45 cos0 sin45 sin0, substitute in the known values 4 cos05 cos(50 45 ) cos50 cos45 + sin50 sin

32 .4. Sum and Difference Identities You do not need to do the problem multiple ways, just the one that seems easiest to you. Example 4: Find the exact value of cos 5π, in radians. Solution: cos 5π cos( π 4 + π ), notice that π 4 π and π π ( π cos 4 + π ) cos π 4 cos π sin π 4 sin π cos π 4 cos π sin π 4 sin π 4 Sum and Difference Identities: Sine To find sin(a + b), use Example, from above: [ π ] sin(a + b) cos (a + b) [( π ) a cos cos ] b ( π a ) cosb + sin ( π a ) sinb Set θ a + b Distribute the negative Difference Formula for cosines sinacosb + cosasinb Co-function Identities In conclusion, sin(a + b) sin a cos b + cos a sin b, which is the sum formula for sine. To obtain the identity for sin(a b): sin(a b) sin[a + ( b)] sin a cos( b) + cos a sin( b) Use the sine sum formula sin(a b) sinacosb cosasinb Use cos( b) cosb, and sin( b) sinb In conclusion, sin(a b) sin a cos b cos a sin b, so, this is the difference formula for sine. Example 5: Find the exact value of sin 5π Solution: Recall that there are multiple angles that add or subtract to equal any angle. Choose whichever formula that you feel more comfortable with. 0 sin 5π ( π sin + π ) sin π π cos sin 5π π + cos sin π

33 Chapter. Trigonometric Identities and Equations Example : Given sinα, where α is in Quadrant II, and sinβ 5, where β is in Quadrant I, find the exact value of sin(α + β). Solution: To find the exact value of sin(α + β), here we use sin(α + β) sinαcosβ + cosαsinβ. The values of sinα and sinβ are known, however the values of cosα and cosβ need to be found. Use sin α + cos α, to find the values of each of the missing cosine values. For cosa : sin α + cos α, substituting sinα transforms to ( ) + cos α cos α or cos α 5 9 cosα ± 5, however, since α is in Quadrant II, the cosine is negative, cosα 5. For cosβ use sin β + cos β and substitute sinβ 5,( ) 5 + cos β cos β or cos β 5 and cosβ ± 4 5 and since β is in Quadrant I, cosβ 4 5 Now the sum formula for the sine of two angles can be found: sin(α + β) 4 ( ) 48 or sin(α + β) 5 Sum and Difference Identities: Tangent To find the sum formula for tangent: sin(a + b) tan(a + b) cos(a + b) sinacosb + sinbcosa cosacosb sinasinb tan(a + b) sinacosb+sinbcosa cosacosb cosacosb sinasinb cosacosb sinacosb cosacosb + sinbcosa cosacosb cosacosb cosacosb sinasinb cosacosb sina cosa + sinb cosb sinasinb cosacosb tana + tanb tanatanb Using tanθ sinθ cosθ Substituting the sum formulas for sine and cosine Divide both the numerator and the denominator by cosacosb Reduce each of the fractions Substitute sinθ cosθ tanθ Sum formula for tangent In conclusion, tan(a + b) tana+tanb tanatanb. Substituting b for b in the above results in the difference formula for tangent: Example 7: Find the exact value of tan85. tan(a b) tana tanb + tanatanb Solution: Use the difference formula for tangent, with

34 .4. Sum and Difference Identities tan(0 45 ) tan0 tan45 + tan0 tan To verify this on the calculator, tan85.7 and.7. Using the Sum and Difference Identities to Verify Other Identities Example 8: Verify the identity cos(x y) sinxsiny cotxcoty + cos(x y) cotxcoty + sinxsiny cosxcosy sinxsiny + sinxsiny sinxsiny cosxcosy sinxsiny + cotxcoty + cotxcoty + Expand using the cosine difference formula. cotangent equals cosine over sine Example 9: Show cos(a + b)cos(a b) cos a sin b Solution: First, expand left hand side using the sum and difference formulas: cos(a + b)cos(a b) (cosacosb sinasinb)(cosacosb + sinasinb) cos acos b sin asin b FOIL, middle terms cancel out Substitute( sin b)forcos b and( cos a)forsin a and simplify. cos a( sin b) sin b( cos a) cos a cos asin b sin b + cos asin b cos a sin b Solving Equations with the Sum and Difference Formulas Just like the section before, we can incorporate all of the sum and difference formulas into equations and solve for values of x. In general, you will apply the formula before solving for the variable. Typically, the goal will be to

35 Chapter. Trigonometric Identities and Equations isolate sinx,cosx, or tanx and then apply the inverse. Remember, that you may have to use the identities in addition to the formulas seen in this section to solve an equation. Example 0: Solve sin(x π) in the interval [0, π). Solution: First, get sin(x π) by itself, by dividing both sides by. sin(x π) sin(x π) Now, expand the left side using the sine difference formula. The sinx when x is π. sinxcosπ cosxsinπ sinx( ) cosx(0) sinx sinx Example : Find all the solutions for cos ( x + π ) in the interval [0,π). Solution: Get the cos ( x + π ) by itself and then take the square root. Now, use the cosine sum formula to expand and solve. ( cos x + π ) ( cos x + π ) ( cos x + π ) The sinx cosxcos π sinxsin π cosx(0) sinx() sinx sinx is in Quadrants III and IV, so x 5π 4 and 7π 4. Points to Consider What are the angles that have 5 and 75 as reference angles?

36 .4. Sum and Difference Identities Are the only angles that we can find the exact sine, cosine, or tangent values for, multiples of π? (Recall that π π would be, making it a multiple of π ) Review Questions. Find the exact value for: a. cos 5π b. cos 7π c. sin45 d. tan75 e. cos45 f. sin 7π. If siny, y is in quad II, and sinz 5. If siny 5 4. Simplify:, z is in quad I find cos(y z), y is in quad III, and sinz 4 5, z is in quad II find sin(y + z) a. cos80 cos0 + sin80 sin0 b. sin5 cos5 + cos5 sin5 5. Prove the identity: cos(m n) sinmcosn cotm + tann. Simplify cos(π + θ) cosθ 7. Verify the identity: sin(a + b)sin(a b) cos b cos a 8. Simplify tan(π + θ) 9. Verify that sin π, using the sine sum formula. 0. Reduce the following to a single term: cos(x + y) cos y + sin(x + y) sin y.. Prove cos(c+d) cos(c d) tanctand +tanctand. Find all solutions to cos ( x + π ), when x is between [0,π).. Solve for all values of x between [0,π) for tan ( x + π ) Find all solutions to sin ( x + π ) ( ) sin x π 4, when x is between [0,π). Review Answers.... cos 5π ( π cos + π ) cos 7π ( 4π cos + π ) ( π cos + π ) 4 4 ( π cos + π ) 4 4 cos π cos π 4 sin π sin π cos π cos π 4 sin π sin π 4 4 sin45 sin( ) sin00 cos45 + cos00 sin

37 Chapter. Trigonometric Identities and Equations tan75 tan( ) tan45 + tan0 tan45 tan0 + sin 7π cos45 cos(5 + 0 ) cos5 cos0 sin5 sin0 ( ) + 4 ( 9π sin + 8π ) ( ) + ( π sin 4 + π ) sin π 4 cos π + cos π 4 sin π If siny 5 and in Quadrant II, then by the Pythagorean Theorem cosy ( + b ). And, if sinz 5 and in Quadrant I, then by the Pythagorean Theorem cosz 4 5 (a + 5 ). So, to find cos(y z) cosycosz + sinysinz and If siny 5 and in Quadrant III, then cosine is also negative. By the Pythagorean Theorem, the second leg is (5 + b ), so cosy. If the sinz 4 5 and in Quadrant II, then the cosine is also negative. By the Pythagorean Theorem, the second leg is (4 + b 5 ), so cos 5. To find sin(y + z), plug this information into the sine sum formula. sin(y + z) sinycosz + cosysinz This is the cosine difference formula, so: cos80 cos0 + sin80 0 cos(80 0 ) cos0. This is the expanded sine sum formula, so: sin5 cos5 + cos5 sin5 sin(5 + 5 ) sin0 4. Step : Expand using the cosine sum formula and change everything into sine and cosine cos(m n) cotm + tann sinmcosn cosmcosn + sinmsinn cosm sinmcosn sinm + sinn cosn Step : Find a common denominator for the right hand side. cosmcosn + sinmsinn sinmcosn The two sides are the same, thus they are equal to each other and the identity is true. 5. cos(π + θ) cosπcosθ sinπsinθ cosθ. Step : Expand sin(a + b) and sin(a b) using the sine sum and difference formulas. sin(a + b) sin(a b) cos b cos a (sinacosb + cosasinb)(sinacosb cosasinb) Step : FOIL and simplify. sin acos b sinacosasinbcosb + sinasinbcosacosb cos asin bsin acos b cosa sin b Step : Substitute ( cos a) for sin a and ( cos b) for sin b, distribute and simplify. ( cos a)cos b cosa ( cos b) cos b cos acos b cos a + cos acos b cos b cos a 5

38 .4. Sum and Difference Identities 7. tan(π + θ) tanπ+tanθ tanπtanθ tanθ tanθ 8. sin π sin( π 4 + π ) 4 sin π 4 cos π 4 cos π 4 sin π This could also be verified by using Step : Expand using the cosine and sine sum formulas. cos(x + y)cosy + sin(x + y)siny (cosxcosy sinxsiny)cosy + (sinxcosy + cosxsiny)siny Step : Distribute cosy and siny and simplify. cosxcos y sinxsinycosy + sinxsinycosy + cosxsin y cosxcos y + cosxsin y cosx(cos y + sin y) } {{ } cosx 0. Step : Expand left hand side using the sum and difference formulas cos(c + d) cos(c d) tanctand + tanctand cosccosd sincsind cosccosd + sincsind tanctand + tanctand Step : Divide each term on the left side by cosccosd and simplify cosccosd cosccosd sincsind cosccosd cosccosd cosccosd + sincsind cosccosd tanctand + tanctand tanctand + tanctand tanctand + tanctand. To find all the solutions, between [0,π), we need ( to expand using the sum formula and isolate the cosx. cos x + π ) ( cos x + π ) ( cos x + π ) ± cosxcos π sinxsin π ± cosx 0 sinx ± sinx ± sinx ± ± This is true when x π 4, π 4, 5π 4, or 7π 4. First, solve for tan(x + π ). ( tan x + π ) + 7 ( tan x + π ) ( tan x + π ) ( tan x + π ) ±

39 Chapter. Trigonometric Identities and Equations Now, use the tangent sum formula to expand for when tan(x + π ). tanx + tan π tanxtan π tanx + tan π ( tanxtan π ) tanx + tanx tanx + tanx tanx tanx This is true when x π or 7π. If the tangent sum formula to expand for when tan(x + π ), we get no solution as shown. tanx + tan π tanxtan π tanx + tan π ( tanxtan π ) tanx + tanx + + tanx + tanx Therefore, the tangent sum formula cannot be used in this case. However, since we know that tan(x + π ) when x + π 5π π or, we can solve for x as follows. x + π 5π x 4π x π x + π π x 0π x 5π Therefore, all of the solutions are x π, π, 7π, 5π. To solve, expand each side: ( sin x + π ) sinxcos π + cosxsin π sinx + cosx ( sin x π ) sinxcos π 4 4 cosxsin π 4 sinx cosx 7

40 .4. Sum and Difference Identities Set the two sides equal to each other: sinx + cosx sinx cosx sinx + cosx sinx cosx sinx sinx cosx cosx ( ) ( sinx cosx ) sinx cosx tanx As a decimal, this is.977, so tan (.977) x,x 90.5 and

41 Chapter. Trigonometric Identities and Equations.5 Double Angle Identities Learning Objectives Use the double angle identities to solve other identities. Use the double angle identities to solve equations. Deriving the Double Angle Identities One of the formulas for calculating the sum of two angles is: sin(α + β) sinαcosβ + cosαsinβ If α and β are both the same angle in the above formula, then sin(α + α) sinαcosα + cosαsinα sinα sinαcosα This is the double angle formula for the sine function. The same procedure can be used in the sum formula for cosine, start with the sum angle formula: cos(α + β) cosαcosβ sinαsinβ If α and β are both the same angle in the above formula, then cos(α + α) cosαcosα sinαsinα cosα cos α sin α This is one of the double angle formulas for the cosine function. Two more formulas can be derived by using the Pythagorean Identity, sin α + cos α. sin α cos α and likewise cos α sin α Using sin α cos α : Using cos α sin α : cosα cos α sin α cosα cos α sin α cos α ( cos α) ( sin α) sin α cos α + cos α sin α sin α cos α sin α 9

42 .5. Double Angle Identities Therefore, the double angle formulas for cosa are: cosα cos α sin α cosα cos α cosα sin α Finally, we can calculate the double angle formula for tangent, using the tangent sum formula: tan(α + β) If α and β are both the same angle in the above formula, then tanα + tanβ tanαtanβ tanα + tanα tan(α + α) tanαtanα tanα tanα tan α Applying the Double Angle Identities Example : If sina 5 and a is in Quadrant II, find sina, cosa, and tana. Solution: To use sina sinacosa, the value of cosa must be found first.. cos a + sin a ( ) 5 cos a + cos a cos a 44,cosa ± 9 However since a is in Quadrant II, cosa is negative or cosa. For cosa, use cos(a) cos a sin a ( ) ( 5 sina sinacosa ) sina ( cos(a) ) ( ) or 9 cos(a) 9 9

43 Chapter. Trigonometric Identities and Equations For tana, use tana tana 5. From above, tana tan a Example : Find cos4θ. tan(a) 5 ( 5 Solution: Think of cos4θ as cos(θ + θ). ) cos4θ cos(θ + θ) cosθcosθ sinθsinθ cos θ sin θ Now, use the double angle formulas for both sine and cosine. For cosine, you can pick which formula you would like to use. In general, because we are proving a cosine identity, stay with cosine. (cos θ ) (sinθcosθ) 4cos 4 θ 4cos θ + 4sin θcos θ 4cos 4 θ 4cos θ + 4( cos θ)cos θ 4cos 4 θ 4cos θ + 4cos θ + 4cos 4 θ 8cos 4 θ 8cos θ + Example : If cotx 4 and x is an acute angle, find the exact value of tanx. Solution: Cotangent and tangent are reciprocal functions, tanx cotx and tanx 4. tanx tanx tan x 4 ( ) Example 4: Given sin(x) and x is in Quadrant I, find the value of sinx. Solution: Using the double angle formula, sin x sin x cos x. Because we do not know cos x, we need to solve for cosx in the Pythagorean Identity, cosx sin x. Substitute this into our formula and solve for sinx. sinx sinxcosx sinx sin x ( ) ( ) sinx sin x 4 9 4sin x( sin x) 4 9 4sin x 4sin 4 x 4

44 .5. Double Angle Identities At this point we need to get rid of the fraction, so multiply both sides by the reciprocal. ( ) sin x 4sin 4 x 9sin x 9sin 4 x 0 9sin 4 x 9sin x + Now, this is in the form of a quadratic equation, even though it is a quartic. Set a sin x, making the equation 9a 9a + 0. Once we have solved for a, then we can substitute sin x back in and solve for x. In the Quadratic Formula, a 9,b 9,c. 9 ± ( 9) 4(9)() (9) 9 ± ± ± 5 8 ± 5 So, a or 5.7. This means that sin x 0.87 or.7 so sinx 0.94 or sinx.57. Example 5: Prove tanθ cosθ sinθ Solution: Substitute in the double angle formulas. Use cosθ sin θ, since it will produce only one term in the numerator. tanθ ( sin θ) sinθcosθ sin θ sinθcosθ sinθ cosθ tanθ Solving Equations with Double Angle Identities Much like the previous sections, these problems all involve similar steps to solve for the variable. trigonometric function, using any of the identities and formulas you have accumulated thus far. Example : Find all solutions to the equation sinx cosx in the interval [0,π] Solution: Apply the double angle formula sin x sin x cos x Isolate the 4

45 Chapter. Trigonometric Identities and Equations sinxcosx cosx sinxcosx cosx cosx cosx sinxcosx cosx 0 cosx(sinx ) 0 Factor out cosx Then cosx 0 or sinx 0 cosx 0 or sinx sinx sinx The values for cosx 0 in the interval [0,π] are x π and x π and the values for sinx in the interval [0,π] are x π and x 5π. Thus, there are four solutions. Example 7: Solve the trigonometric equation sin x sin x such that ( π x < π) Solution: Using the sine double angle formula: sinx sinx sinxcosx sinx sinxcosx sinx 0 sinx(cosx ) 0 cosx 0 cosx sinx 0 x 0, π cosx x π, π Example 8: Find the exact value of cosx given cosx 4 if x is in the second quadrant. Solution: Use the double-angle formula with cosine only. 4

46 .5. Double Angle Identities cosx cos x ( cosx ) 4 ( ) 9 cosx 9 ( ) 8 cosx 9 cosx cosx Example 9: Solve the trigonometric equation 4sinθcosθ over the interval [0,π). Solution: Pull out a from the left-hand side and this is the formula for sinx. 4sinθcosθ (sinθcosθ) (sinθcosθ) sinθ sinθ sinθ The solutions for θ are π, π, 7π, 8π, dividing each of these by, we get the solutions for θ, which are π, π, 7π, 8π. Points to Consider Are there similar formulas that can be derived for other angles? Can technology be used to either solve these trigonometric equations or to confirm the solutions? Review Questions 44. If sinx 4 5 and x is in Quad II, find the exact values of cosx,sinx and tanx. Find the exact value of cos 5 sin 5. Verify the identity: cosθ 4cos θ cosθ 4. Verify the identity: sint tant tant cost 5. If sinx 9 4 and x is in Quad III, find the exact values of cosx,sinx and tanx. Find all solutions to sinx + sinx 0 if 0 x < π 7. Find all solutions to cos x cosx 0 if 0 x < π 8. If tanx 4 and 0 < x < 90, use the double angle formulas to determine each of the following: a. tanx b. sinx