POP QUIZ Lesson - Trigonometric Identities IB Math HL () Solve 5 = x 3 () Solve 0 = x x 6 (3) Solve = /x (4) Solve 4 = x (5) Solve sin(θ) = (6) Solve x x x x (6) Solve x + = (x + ) (7) Solve 4(x ) = (x )(x + ) (x ) /9/04 HL Math - Santowski /9/04 HL Math - Santowski POP QUIZ (A) Review of Equations FACTOR THE FOLLOWING EXPRESSIONS: (a) x (b) x y (c) x + x + (d) x x (e) x x + (f) xy x An equation is an algebraic statement that is true for only several values of the variable The linear equation 5 = x 3 is only true for..? The quadratic equation 0 = x x 6 is true only for? The trig equation sin(θ) = is true for? The reciprocal equation = /x is true only for.? The root equation 4 = x is true for..? 3 /9/04 HL Math - Santowski 4 /9/04 HL Math - Santowski (A) Review of Equations (B) Introduction to Identities An equation is an algebraic statement that is true for only several values of the variable The linear equation 5 = x 3 is only true for the x value of 4 The quadratic equation 0 = x x 6 is true only for x = - and x = 3 (i.e. 0 = (x 3)(x + )) The trig equation sin(θ) = is true for several values like 90, 450, -70, etc The reciprocal equation = /x is true only for x = ½ The root equation 4 = x is true for one value of x = 6 Now imagine an equation like x + = (x + ) and we ask ourselves the same question for what values of x is it true? Now 4(x ) = (x )(x + ) (x ) and we ask ourselves the same question for what values of x is it true? x What about x x x 5 /9/04 HL Math - Santowski /9/04 HL Math - Santowski 6 TF.0.8 - Trigonometric Identities
(B) Introduction to Identities (B) Introduction to Identities Now imagine an equation like x + = (x + ) and we ask ourselves the same question for what values of x is it true? We can actually see very quickly that the right side of the equation expands to x +, so in reality we have an equation like x + = x + But the question remains for what values of x is the equation true?? Since both sides are identical, it turns out that the equation is true for ANY value of x we care to substitute! So we simply assign a slightly different name to these special equations we call them IDENTITIES because they are true for ALL values of the variable! For example, 4(x ) = (x )(x + ) (x ) Is this an identity (true for ALL values of x) or simply an equation (true for one or several values of x)??? The answer lies with our mastery of fundamental algebra skills like expanding and factoring so in this case, we can perform some algebraic simplification on the right side of this equation RS = (x 4) (x 4x + 4) RS = -4 + 4x 4 RS = 4x 8 RS = 4(x ) So yes, this is an identity since we have shown that the sides of the equation are actually the same expression 7 /9/04 HL Math - Santowski /9/04 HL Math - Santowski 8 (C) Domain of Validity When two algebraic expressions (the LS and RS of an equation) are equal to each other for specific values of the variable, then the equation is an IDENTITY Quotient Identity This equation is an identity as long as the variables make the statement defined, in other words, as long as the variable is in the domain of each expression. The set of all values of the variable that make the equation defined is called the domain of validity of the identity. (It is really the same thing as the domain for both sides of the identity /9/04 HL Math - Santowski 9 0 /9/04 HL Math - Santowski Recall our definitions for sin(θ) = o/h, cos(θ) = a/h and tan(θ) = o/a So now one trig identity can be introduced if we take sin(θ) and divide by cos(θ), what do we get? sin(θ) = o/h = o = tan(θ) cos(θ) a/h a So the tan ratio is actually a quotient of the sine ratio divided by the cosine ratio What would the cotangent ratio then equal? So the tan ratio is actually a quotient of the sine ratio divided by the cosine ratio We can demonstrate this in several ways we can substitute any value for θ into this equation and we should notice that both sides always equal the same number Or we can graph f(θ) = sin(θ)/cos(θ) as well as f(θ) = tan(θ) and we would notice that the graphs were identical This identity is called the QUOTIENT identity /9/04 HL Math - Santowski /9/04 HL Math - Santowski TF.0.8 - Trigonometric Identities
Pythagorean Identity Another key identity is called the Pythagorean identity In this case, since we have a right triangle, we apply the Pythagorean formula and get x + y = r Now we simply divide both sides by r 3 /9/04 HL Math - Santowski /9/04 HL Math - Santowski 4 Now we simply divide both sides by r and we get x /r + y /r = r /r Upon simplifying, (x/r) + (y/r) = But x/r = cos(θ) and y/r = sin(θ) so our equation becomes (cos(θ)) + (sin(θ)) = Or rather cos (θ) + sin (θ) = Which again can be demonstrated by substitution or by graphing Now we simply divide both sides by x and we get x /x + y /x = r /x Upon simplifying, + (y/x) = (r/x) But y/x = tan(θ) and r/x = sec(θ) so our equation becomes + (tan(θ)) = (sec(θ)) Or rather + tan (θ) = sec (θ) Which again can be demonstrated by substitution or by graphing 5 /9/04 HL Math - Santowski 6 /9/04 HL Math - Santowski (G) Simplifying Trig Expressions Now we simply divide both sides by y and we get x /y + y /y = r /y Upon simplifying, (x/y) + = (r/y) But x/y = cot(θ) and r/y = csc(θ) so our equation becomes (cot(θ)) + = (csc(θ)) Or rather + cot (θ) = csc (θ) Simplify the following expressions: ( a) 3 ( b) ( c) ( d) Which again can be demonstrated by substitution or by graphing /9/04 HL Math - Santowski 7 /9/04 HL Math - Santowski 8 TF.0.8 - Trigonometric Identities 3
(G) Simplifying Trig Expressions Prove tan(x) cos(x) = sin(x) Prove tan (x) = sin (x) cos - (x) Prove Prove tan x tan x /9/04 HL Math - Santowski 9 /9/04 HL Math - Santowski 0 Prove tan(x) cos(x) = sin(x) LS tan x LS LS LS RS Prove tan (x) = sin (x) cos - (x) RS sin RS sin x RS sin x x cos sin x RS cos x RS RS tan x RS LS x cos x /9/04 HL Math - Santowski /9/04 HL Math - Santowski Prove tan x tan x LS tan x tan x LS LS LS LS LS LS RS /9/04 3 HL Math - Santowski Prove sin x LS x LS cos ( )( ) LS ( ) LS LS RS /9/04 HL Math - Santowski 4 TF.0.8 - Trigonometric Identities 4
Examples (D) Solving Trig Equations with Substitutions Identities tan cos 0 for /9/04 HL Math - Santowski 5 /9/04 HL Math - Santowski 6 (D) Solving Trig Equations with Substitutions Identities But tan cos 0 for tan x So tan x So we make a substitution and simplify our equation sin 0 for 3, sin x for x /9/04 HL Math - Santowski 7 /9/04 HL Math - Santowski 8 sin x Now, one option is: for x x for x the following (a) 0 (b) (c) tan x for x for x for x /9/04 HL Math - Santowski 9 /9/04 HL Math - Santowski 30 TF.0.8 - Trigonometric Identities 5
(F) Example Since - cos x = sin x is an identity, what about? /9/04 HL Math - Santowski 3 TF.0.8 - Trigonometric Identities 6