Math 302 Module 6. Department of Mathematics College of the Redwoods. June 17, 2011

Transcription

1 Math 302 Module 6 Department of Mathematics College of the Redwoods June 17, 2011

2 Contents 6 Radical Expressions 1 6a Square Roots... 2 Introduction to Radical Notation... 2 Approximating Square Roots b Factoring... 8 Simplifying Radical Expressions... 8 Simple Form c Pythagorean Theorem The Pythagorean Theorem Proof of the Pythagorean Theorem Applications ii

3 Module 6 Radical Expressions 1

4 2 MODULE 6. RADICAL EXPRESSIONS 6a Square Roots Introduction to Radical Notation We know how to square a number. For example: 5 2 =25 ( 5) 2 =25 Taking the square root of a number is the opposite of squaring. The nonnegative square root of 25 is 5. The negative square root of 25 is 5. Thus, when searching for a square root of a number, we are searching for number whose square is equal to our number. EXAMPLE 1. Find the square roots of 81. Solution: We are looking for a number whose square is 81. Because 9 2 = 81, the nonnegative square root of 81 is 9. Because ( 9) 2 = 81, the negative square root of 81 is 9. Hence, 81 has two square roots, 9 and9. EXAMPLE 2. Find the square roots of 0. Solution: We are looking for a number whose square is 0. Because 0 2 = 0, the nonnegative square root of 0 is 0. No other number squared will equal zero. Hence, zero has exactly one square root, namely zero. EXAMPLE 3. Find the square roots of 36.

5 6A. SQUARE ROOTS 3 Solution: We are looking for a number whose square is 36. However, every time you square a real number, the result is never negative. Hence, 36 has no real square roots. 1 The introductions in Examples 1, 2, and 3 lead to the following definition. Defining the square roots of a number. The solutions of x 2 = a are called square roots of a. Case: a>0. The equation x 2 = a has two real solutions, namely x = ± a. The notation a calls for the nonegative square root. The notation a calls for the egative square root. Case: a = 0. The equation x 2 = a has exactly one solution, namely x =0. Case: a<0. The equation x 2 = a has no real solutions. EXAMPLE 4. Solve x 2 =9forx, then simplify 9and 9. Solution: There are two numbers whose square equal 9, namely 3 and3. Hence: x 2 =9 Original equation. x = ±3 Two answers: ( 3) 2 =9and3 2 =9. Writing x = ±3 is a shortcut for writing x = 3 orx =3. Next: 9 calls for the nonnegative square root of 9. Hence: 9=3 9 calls for the negative square root of 9. Hence: 9= 3 1 When we say that 36 has no real square roots, we mean there are no real numbers that are square roots of 36. The reason we emphasize the word real in this situation is the fact there 36 does have two square roots that are elements of the complex numbers, asetofnumbersthatareusuallyintroducedinadvancedcourses such as college algebra or trigonometry.

6 4 MODULE 6. RADICAL EXPRESSIONS EXAMPLE 5. Solve x 2 =0forx, then simplify 0. Solution: There is only one number whose square equals 0, namely 0. Hence: x 2 =0 Original equation. x =0 One answer: (0) 2 =0. Thus, the only solution of x 2 = 0 is x =0. Next: 0 calls for the nonnegative square root of 0. Hence: 0=0 EXAMPLE 6. Solve x 2 = 4 forx, then simplify 4. Solution: You cannot square a real number and get negative. Hence, x 2 = 4 has no real solutions. Further: 4 calls for the nonnegative square root of 4. Because you cannot square a real number and get 4, there is no nonegative square root of 4. EXAMPLE 7. Simplify each of the following: a) 121 b) 225 c) 100 d) 324 Solution: Remember, the notation a calls for the nonnegative square root of a, while the notation a calls for the negative square root of a. Because 11 2 = 121, the nonnegative square root of 121 is 11. Thus: 121 = 11 Because ( 15) 2 = 225, the negative square root of 225 is 15. Thus: 225 = 15 You cannot square a real number and get 100. Therefore, 100 is not arealnumber.

7 6A. SQUARE ROOTS 5 Because ( 18) 2 = 324, the negative square root of 324 is 18. Thus: 324 = 18 Squaring undos taking the square root. Squaring square roots. If a>0, then both a and a are solutions of x 2 = a. Consequently, if we substitute each of them into the equation x 2 = a, we get: ( a ) 2 = a and ( a ) 2 = a EXAMPLE 8. Simplify each of the following expressions: a) ( 5) 2 b) ( 7) 2 c) ( 11) 2 Solution: We ll handle each case carefully. a) Because 5 is a solution of x 2 = 5, if we square 5, we should get 5. ( 5) 2 =5 b) Because 7 is a solution of x 2 = 7, if we square 7, we should get 7. ( 7) 2 =7 c) Because x 2 = 11 has no real answers, 11 is not a real number. Advanced courses such as college algebra or trigonometry will introduce the complex number system and show how to handle this expression. At this level, we comment that 11 is not a real number and cease and desist. Approximating Square Roots The squares in the List of Squares are called perfect squares. Each is the square of a whole number. Not all numbers are perfect squares. For example, in the case of 24, there is no whole number whose square is equal to 24. However, this does not prevent 24 from being a perfectly good number. We can use the List of Squares to find decimal approximations when the radicand is not a perfect square.

8 6 MODULE 6. RADICAL EXPRESSIONS Estimate: 83 List of Squares x x EXAMPLE 9. Estimate 24 by guessing. Solution. From the List of Squares, note that 24 lies betwen 16 and 25, so 24 will lie between 4 and 5, with 24 much closer to 5 than it is to Let s guess As a check, let s square 4.8. (4.8) 2 =(4.8)(4.8) = Not quite 24! Clearly, 24 must be a little bit bigger than 4.8. Just for fun, here is a decimal approximation of 24 that is accurate to 1000 places, courtesy of If you were to multiply this number by itself (square the number), you would get a number that is extremely close to 24, but it would not be exactly 24. There would still be a little discrepancy. Answer: 9.1

9 6B. FACTORING 7 6b Factoring Simplifying Radical Expressions Let s begin by comparing two mathematical expressions = 3 4 = = 144 =12 Note that both 9 16 and 9 16 equal 12. Hence, 9 16 = Let s look at another example. 4 9=2 3 =6 4 9= 36 =6 Note that both 4 9and 4 9equal6. Hence, 4 9= 4 9. It appears that a pattern is forming, namely: a b = ab Multiplication property of radicals. If a 0andb 0, then: a b = ab EXAMPLE 1. Simplify each of the following expressions as much as possible: a) 3 11 b) 12 3 c) 2 13 Solution: In each case, use the property a b = ab. That is, multiply the two numbers under the square root sign, placing the product under a single square root. a) 3 11 = 3 11 = 33 b) 12 3= 12 3 = 36 =6 c) 2 13 = 2 13 = 26

10 8 MODULE 6. RADICAL EXPRESSIONS Simple Form We can also use the property a b = ab in reverse to factor out a perfect square. For example: 50 = 25 2 Factor out a perfect square. =5 2 Simplify: 25 = 5. The expression 5 2 is said to be in simple radical form. Like reducing a fraction to lowest terms, always look to factor out a perfect square when possible. Simple radical form. When all perfect squares have been factored out of a radical, then the expression is said to be in simple radical form. EXAMPLE 2. Place 8 in simple form. Solution: From 8, we can factor out a perfect square, in this case 4. 8= 4 2 Factor out a perfect square. =2 2 Simplify: 4=2. Sometimes, after factoring out a perfect square, you can still factor out another perfect square. EXAMPLE 3. Place 72 in simple form. Solution: From 72, we can factor out a perfect square, in this case = 9 8 Factor out a perfect square. =3 8 Simplify: 9=3. However, from 8 we can factor out another perfect square, in this case 4. =3 4 2 Factor out another perfect square. =3 2 2 Simplify: 4=2. =6 2 Multiply: 3 2=6.

11 6B. FACTORING 9 Alternate solution. We can simplify the process by noting that we can factor out a 36 from 72 to start the process. 72 = 36 2 Factor out a perfect square. =6 2 Simplify: 36 = 6. Although the second solution is more efficient, the first solution is still mathematically correct. The point to make here is that we must continue to factor out a perfect square whenever possible. Our answer is not in simple form until we can no longer factor out a perfect square.

12 10 MODULE 6. RADICAL EXPRESSIONS 6c Pythagorean Theorem The Pythagorean Theorem An angle that measures 90 degrees is called a right angle. If one of the angles of a triangle is a right angle, then the triangle is called a right triangle. It is traditional to mark the right angle with a little square (see Figure 6.1). B c a A b C Figure 6.1: Right triangle ABC has a right angle at vertex C. Right triangle terminology. The longest side of the right triangle, the side directly opposite the right angle, is called the hypotenuse of the right triangle. The remaining two sides of the right triangle are called the legs of the right triangle. Proof of the Pythagorean Theorem Each side of the square in Figure 6.2 has been divided into two segments, one of length a, the other of length b. We can find the total area of the square by squaring any one of the sides of the square. A =(a + b) 2 A = a 2 +2ab + b 2 Square a side to find area. Squaring a binomial pattern. Thus, the total area of the square is A = a 2 +2ab + b 2. A second approach to finding the area of the square is to sum the areas of the geometric parts that make up the square. We have four congruent right triangles, shaded in light red, with base a and height b. The area of each of these triangles is found by taking one-half times the base times the height; i.e., the area of each triangles is (1/2)ab. In the interior, we have a smaller square

13 6C. PYTHAGOREAN THEOREM 11 b a a c c b b c c a a b Figure 6.2: Proving the Pythagorean Theorem. with side c. Its area is found by squaring its side; i.e., the area of the smaller square is c 2. The total area of the square is the sum of its parts, one smaller square and four congruent triangles. That is: A = c 2 +4 A = c 2 +2ab ( ) 1 2 ab Adding the area of the interior square and the area of four right triangles. Simplify: 4((1/2)ab) =2ab. The two expressions, a 2 +2ab + b 2 and c 2 +2ab, bothrepresentthetotalarea of the large square. Hence, they must be equal to one another. a 2 +2ab + b 2 = c 2 +2ab a 2 + b 2 = c 2 Each side of this equation represents the area of the large square. Subtract 2ab from both sides. The last equation, a 2 + b 2 = c 2, is called the Pythagorean Theorem. The Pythagorean Theorem. If a and b are the legs of a right triangle and c is its hypotenuse, then: a 2 + b 2 = c 2 We say The sum of the squares of the legs of a right triangle equals the square of its hypotenuse. Good hint. Note that the hypotenuse sits by itself on one side of the equation a 2 + b 2 = c 2. The legs of the hypotenuse are on the other side. Let s put the Pythagorean Theorem to work.

14 12 MODULE 6. RADICAL EXPRESSIONS EXAMPLE 1. Find the missing side of the right triangle shown below. c 3 4 Solution: First, write out the Pythagorean Theorem, then substitute the given values in the appropriate places. a 2 + b 2 = c 2 Pythagorean Theorem. (4) 2 +(3 2 )=c 2 Substitute: 4 for a, 3forb = c 2 Square: 4 2 =16,3 2 =9. 25 = c 2 Add: = 25. The equation c 2 = 25 has two real solutions, c = 5 andc = 5. However, in this situation, c represents the length of the hypotenuse and must be a positive number. Hence: c = 5 Nonnegative square root. Thus, the length of the hypotenuse is 5. EXAMPLE 2. An isosceles right triangle has a hypotenuse of length 8. Find the lengths of the legs. Solution: In general, an isosceles triangle is a triangle with two equal sides. In this case, an isosceles right triangle has two equal legs. We ll let x represent the length of each leg. x 8 x

15 6C. PYTHAGOREAN THEOREM 13 Use the Pythagorean Theorem, substituting x for each leg and 8 for the hypotenuse. a 2 + b 2 = c 2 x 2 + x 2 =8 2 2x 2 =64 Pythagorean Theorem. Substitute: x for a, x for b, 8forc. Combine like terms: x + x =2x. x 2 =32 Divide both sides by 2. The equation x 2 = 32 has two real solutions, x = 32 and x = 32. However, in this situation, x represents the length of each leg and must be a positive number. Hence: x = 32 Nonnegative square root. Remember, your final answer must be in simple form. We must factor out a perfect square when possible. x = 16 2 Factor out a perfect square. x =4 2 Simplify: 16 = 4. Thus, the length of each leg is 4 2. Applications EXAMPLE 3. A ladder 20 feet long leans against the garage wall. If the base of the ladder is 8 feet from the garage wall, how high up the garage wall does the ladder reach? Find an exact answer, then use your calculator to round your answer to the nearest tenth of a foot. Solution: As always, we obey the Requirements for Word Problem Solutions. 1. Set up a variable dictionary. We ll create a well-marked diagram for this purpose, letting h represent the distance between the base of the garage wall and the upper tip of the ladder.

16 14 MODULE 6. RADICAL EXPRESSIONS 20 ft h 8ft 2. Set up an equation. Using the Pythagorean Theorem, we can write: h 2 =20 2 Pythagorean Theorem h 2 =400 Square: 8 2 =64and20 2 = Solve the equation. h 2 =336 Subtract 64 from both sides. h = 336 h will be the nonnegative square root. h = Factor out a perfect square. h =4 21 Simplify: 16 = Answer the question. The ladder reaches 4 21 feet up the wall. 5. Look back. 20 ft 4 21 ft 8ft

17 6C. PYTHAGOREAN THEOREM 15 Using the Pythagorean Theorem: ( ) 2 21 = (4) 2 ( 21 ) 2 = (16) (21) = = = 400