9.7 Using Trigonometric Identities EEssentialssential QQuestionuestion How can you verify a trigonometric identity? Writing a Trigonometric Identity Work with a partner. In the fi gure, the point y (x, y) is on a circle of radius c with center at the origin. (x, y) a. Write an equation that relates a, b, and c. b. Write expressions for the sine and cosine c ratios of angle θ. b c. Use the results from parts (a) and (b) to fi nd the sum of sin2θ and cos2θ. What do θ you observe? a x d. Complete the table to verify that the identity you wrote in part (c) is valid for angles (of your choice) in each of the four quadrants. θ sin2 θ cos2 θ sin2 θ + cos2 θ QI QII QIII QIV Writing Other Trigonometric Identities Work with a partner. The trigonometric identity you derived in Exploration 1 is REASONING called a Pythagorean identity. There are two other Pythagorean identities. To derive ABSTRACTLY them, recall the four relationships: θ θ To be profi cient in math, θ = —sin θ = —cos tan θ cot θ you need to know and cos sin fl exibly use different 1 1 sec θ = — csc θ = — properties of operations cos θ sin θ and objects. a. Divide each side of the Pythagorean identity you derived in Exploration 1 by cos2θ and simplify. What do you observe? b. Divide each side of the Pythagorean identity you derived in Exploration 1 by sin2θ and simplify. What do you observe? CCommunicateommunicate YourYour AnswerAnswer 3. How can you verify a trigonometric identity? 4. Is sin θ = cos θ a trigonometric identity? Explain your reasoning. 5. Give some examples of trigonometric identities that are different than those in Explorations 1 and 2. Section 9.7 Using Trigonometric Identities 513 hhsnb_alg2_pe_0907.inddsnb_alg2_pe_0907.indd 513513 22/5/15/5/15 11:53:53 PPMM 9.7 Lesson WWhathat YYouou WWillill LLearnearn Use trigonometric identities to evaluate trigonometric functions and simplify trigonometric expressions. Core VocabularyVocabulary Verify trigonometric identities. trigonometric identity, p. 514 Previous Using Trigonometric Identities unit circle Recall that when an angle θ is in standard y position with its terminal side intersecting the unit circle at (x, y), then x = cos θ and (cos θθ , sin ) = (x, y) r = 1 y = sin θ. Because (x, y) is on a circle θ centered at the origin with radius 1, it x STUDY TIP follows that Note that sin2 θ represents x2 + y2 = 1 θ 2 2 θ (sin ) and cos and represents (cos θ)2. cos2 θ + sin2 θ = 1. The equation cos2 θ + sin2 θ = 1 is true for any value of θ. A trigonometric equation that is true for all values of the variable for which both sides of the equation are defi ned is called a trigonometric identity. In Section 9.1, you used reciprocal identities to fi nd the values of the cosecant, secant, and cotangent functions. These and other fundamental trigonometric identities are listed below. CCoreore CConceptoncept Fundamental Trigonometric Identities Reciprocal Identities 1 1 1 csc θ = — sec θ = — cot θ = — sin θ cos θ tan θ Tangent and Cotangent Identities sin θ cos θ tan θ = — cot θ = — cos θ sin θ Pythagorean Identities sin2 θ + cos2 θ = 1 1 + tan2 θ = sec2 θ 1 + cot2 θ = csc2 θ Cofunction Identities π π π sin — − θ = cos θ cos — − θ = sin θ tan — − θ = cot θ ( 2 ) ( 2 ) ( 2 ) Negative Angle Identities sin(−θ) = −sin θ cos(−θ) = cos θ tan(−θ) = −tan θ In this section, you will use trigonometric identities to do the following. • Evaluate trigonometric functions. • Simplify trigonometric expressions. • Verify other trigonometric identities. 514 Chapter 9 Trigonometric Ratios and Functions hhsnb_alg2_pe_0907.inddsnb_alg2_pe_0907.indd 514514 22/5/15/5/15 11:54:54 PPMM Finding Trigonometric Values 4 π Given that sin θ = — and — < θ < π, fi nd the values of the other fi ve trigonometric 5 2 functions of θ. SOLUTION Step 1 Find cos θ. sin2 θ + cos2 θ = 1 Write Pythagorean identity. 4 2 4 — + cos2 θ = 1 Substitute — for sin θ. ( 5) 5 4 2 4 2 cos2 θ = 1 − — Subtract — from each side. ( 5) ( 5) 9 cos2 θ = — Simplify. 25 3 cos θ = ± — Take square root of each side. 5 3 cos θ = − — Because θ is in Quadrant II, cos θ is negative. 5 Step 2 Find the values of the other four trigonometric functions of θ using the values of sin θ and cos θ. 4 3 — − — sin θ 5 4 cos θ 5 3 tan θ = — = — = − — cot θ = — = — = − — cos θ 3 3 sin θ 4 4 − — — 5 5 1 1 5 1 1 5 csc θ = — = — = — sec θ = — = — = − — sin θ 4 4 cos θ 3 3 — − — 5 5 Simplifying Trigonometric Expressions π Simplify (a) tan — − θ sin θ and (b) sec θ tan2 θ + sec θ. ( 2 ) SOLUTION π a. tan — − θ sin θ = cot θ sin θ Cofunction identity ( 2 ) cos θ = — (sin θ) Cotangent identity ( sin θ ) = cos θ Simplify. b. sec θ tan2 θ + sec θ = sec θ(sec2 θ − 1) + sec θ Pythagorean identity = sec3 θ − sec θ + sec θ Distributive Property = sec3 θ Simplify. MMonitoringonitoring PProgressrogress Help in English and Spanish at BigIdeasMath.com 1 π 1. Given that cos θ = — and 0 < θ < — , fi nd the values of the other fi ve 6 2 trigonometric functions of θ. Simplify the expression. tan x csc x 2. sin x cot x sec x 3. cos θ − cos θ sin2 θ 4. — sec x Section 9.7 Using Trigonometric Identities 515 hhsnb_alg2_pe_0907.inddsnb_alg2_pe_0907.indd 515515 22/5/15/5/15 11:54:54 PPMM Verifying Trigonometric Identities You can use the fundamental identities from this chapter to verify new trigonometric identities. When verifying an identity, begin with the expression on one side. Use algebra and trigonometric properties to manipulate the expression until it is identical to the other side. Verifying a Trigonometric Identity sec2 θ − 1 Verify the identity — = sin2 θ. sec2 θ SOLUTION sec2 θ − 1 sec2 θ 1 — = — − — Write as separate fractions. sec2 θ sec2 θ sec2 θ 1 2 = 1 − — Simplify. ( sec θ) = 1 − cos2 θ Reciprocal identity = sin2 θ Pythagorean identity Notice that verifying an identity is not the same as solving an equation. When verifying an identity, you cannot assume that the two sides of the equation are equal because you are trying to verify that they are equal. So, you cannot use any properties of equality, such as adding the same quantity to each side of the equation. Verifying a Trigonometric Identity cos x Verify the identity sec x + tan x = — . 1 − sin x LOOKING FOR SOLUTION STRUCTURE 1 sec x + tan x = — + tan x Reciprocal identity To verify the identity, you cos x − x must introduce 1 sin 1 sin x = — + — Tangent identity into the denominator. cos x cos x Multiply the numerator 1 + sin x and the denominator by = — Add fractions. 1 − sin x so you get an cos x equivalent expression. 1 + sin x 1 − sin x 1 − sin x = — ⋅ — Multiply by — . cos x 1 − sin x 1 − sin x 1 − sin2 x = —— Simplify numerator. cos x(1 − sin x) cos2 x = —— Pythagorean identity cos x(1 − sin x) cos x = — Simplify. 1 − sin x MMonitoringonitoring PProgressrogress Help in English and Spanish at BigIdeasMath.com Verify the identity. 5. cot(−θ) = −cot θ 6. csc2 x(1 − sin2 x) = cot2 x 7. cos x csc x tan x = 1 8. (tan2 x + 1)(cos2 x − 1) = −tan2 x 516 Chapter 9 Trigonometric Ratios and Functions hhsnb_alg2_pe_0907.inddsnb_alg2_pe_0907.indd 516516 22/5/15/5/15 11:54:54 PPMM 9.7 Exercises Dynamic Solutions available at BigIdeasMath.com VVocabularyocabulary andand CoreCore ConceptConcept CheckCheck 1. WRITING Describe the difference between a trigonometric identity and a trigonometric equation. 2. WRITING Explain how to use trigonometric identities to determine whether sec(−θ) = sec θ or sec(−θ) = −sec θ. MMonitoringonitoring ProgressProgress andand ModelingModeling withwith MathematicsMathematics In Exercises 3–10, fi nd the values of the other fi ve ERROR ANALYSIS In Exercises 21 and 22, describe and trigonometric functions of θ. (See Example 1.) correct the error in simplifying the expression. 1 π 3. sin θ = — , 0 < θ < — 21. 3 2 ✗ 1 − sin2 θ = 1 − (1 + cos2 θ) 7 3π = 1 − 1 − cos2 θ 4. sin θ = − — , π < θ < — 10 2 = −cos2 θ 3 π 5. tan θ = − — , — < θ < π 7 2 22. cos x 1 2 π tan x csc x = — — 6. cot θ = − — , — < θ < π ✗ sin x ⋅sin x 5 2 cos x = — 5 3π sin2 x 7. cos θ = − — , π < θ < — 6 2 9 3π 8. sec θ = — , — < θ < 2π In Exercises 23–30, verify the identity. (See Examples 3 4 2 and 4.) 3π 9. cot θ = −3, — < θ < 2π 23. sin x csc x = 1 24. tan θ csc θ cos θ = 1 2 π 5 3π − = 10. csc θ = − — , π < θ < — 25. cos — x cot x cos x 3 2 ( 2 ) π − = In Exercises 11–20, simplify the expression. 26. sin — x tan x sin x (See Example 2.) ( 2 ) π — − θ + θ + 2 θ cos 1 11. sin x cot x 12. cos (1 tan ) ( 2 ) sin2(−x) 27. —— = 1 28. — = cos2 x − −θ 2 sin(−θ) cos2 x 1 sin( ) tan x 13. — 14. — cos(−θ) cot2 x 1 + cos x sin x 29. — + — = 2 csc x π sin x 1 + cos x — − cos x π ( 2 ) − θ θ sin x 15. — 16. sin — sec 30. —— = csc x + cot x csc x ( 2 ) 1 − cos(−x) csc2 x − cot2 x cos2 x tan2(−x) − 1 17.