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- 2. Trigonometric Identities We know that an equation is a statement that two mathematical expressions are equal.
- 3. Trigonometric Identities A trigonometric identity is an identity involving trigonometric functions. We begin by listing some
- 4. Simplifying Trigonometric Expressions Identities enable us to write the same expression in different ways. It is
- 5. Example 1 – Simplifying a Trigonometric Expression Simplify the expression cos t + tan t sin
- 6. Proving Trigonometric Identities
- 7. Proving Trigonometric Identities Many identities follow from the fundamental identities. In the examples that follow, we
- 8. Proving Trigonometric Identities Thus the equation sin x + cos x = 1 is not an
- 9. Proving Trigonometric Identities
- 10. Example 2 – Proving an Identity by Rewriting in Terms of Sine and Cosine Consider the
- 11. Example 2 – Solution = 1 – cos2θ = sin2θ = RHS (b) We graph each
- 12. Proving Trigonometric Identities In Example 2 it isn’t easy to see how to change the right-hand
- 13. Proving Trigonometric Identities In Example 3 we introduce “something extra” to the problem by multiplying the
- 14. Example 3 – Proving an Identity by Introducing Something Extra Verify the identity = sec u
- 15. Example 3 – Solution = = = = = sec u + tan u cont’d Expand
- 16. Proving Trigonometric Identities Here is another method for proving that an equation is an identity. If
- 17. Example 4 – Proving an Identity by Working with Both Sides Separately Verify the identity Solution:
- 18. Example 4 – Solution RHS = = = secθ + 1 It follows that LHS =
- 19. Proving Trigonometric Identities We conclude this section by describing the technique of trigonometric substitution, which we
- 20. Example 5 – Trigonometric Substitution Substitute sinθ for x in the expression , and simplify. Assume
- 21. 7.2 Addition and Subtraction Formulas
- 22. Addition and Subtraction Formulas We now derive identities for trigonometric functions of sums and differences.
- 23. Example 1 – Using the Addition and Subtraction Formulas Find the exact value of each expression.
- 24. Example 1 – Solution (b) Since the Subtraction Formula for Cosine gives cos = cos =
- 25. Example 2 – Proving a Cofunction Identity Prove the cofunction identity cos = sin u. Solution:
- 26. Addition and Subtraction Formulas The cofunction identity in Example 3, as well as the other cofunction
- 27. Example 3 – An identity from Calculus If f (x) = sin x, show that Solution:
- 28. Example 3 – Solution cont’d Factor Separate the fraction
- 29. Evaluating Expressions Involving Inverse Trigonometric Functions
- 30. Evaluating Expressions Involving Inverse Trigonometric Functions Expressions involving trigonometric functions and their inverses arise in calculus.
- 31. Example 4 – Simplifying an Expression Involving Inverse Trigonometric Functions Write sin(cos–1 x + tan–1 y)
- 32. Example 4 – Solution From the triangles we have sin θ = cos φ = sin
- 33. Example 4 – Solution cont’d From triangles
- 34. Expressions of the Form A sin x + B cos x
- 35. Expressions of the Form A sin x + B cos x We can write expressions of
- 36. Expressions of the Form A sin x + B cos x We are able to do
- 37. Expressions of the Form A sin x + B cos x We need a number φ
- 38. Expressions of the Form A sin x + B cos x With this φ we have
- 39. Example 5 – A Sum of Sine and Cosine Terms Express 3 sin x + 4
- 40. Example 5 – Graphing a Trigonometric Function Write the function f (x) = –sin 2x +
- 41. Example 5 – Solution Thus φ = 2π /3. By the preceding theorem we can write
- 42. Example 5 – Solution We see that the graph is a sine curve with amplitude 2,
- 43. 7.3 Double-Angle, Half-Angle, and Product-Sum Formulas
- 44. Double-Angle, Half-Angle, and Product-Sum Formulas The identities we consider in this section are consequences of the
- 45. Double-Angle Formulas The formulas in the following box are immediate consequences of the addition formulas.
- 46. Example 1– A Triple-Angle Formula Write cos 3x in terms of cos x. Solution: cos 3x
- 47. Example 1 – Solution = 2 cos3 x – cos x – 2 cos x (1
- 48. Double-Angle Formulas Example 2 shows that cos 3x can be written as a polynomial of degree
- 49. Half-Angle Formulas
- 50. Half-Angle Formulas The following formulas allow us to write any trigonometric expression involving even powers of
- 51. Example 2 – Lowering Powers in a Trigonometric Expression Express sin2 x cos2 x in terms
- 52. Example 2 – Solution Another way to obtain this identity is to use the Double-Angle Formula
- 53. Half-Angle Formulas
- 54. Example 3 – Using a Half-Angle Formula Find the exact value of sin 22.5°. Solution: Since
- 55. Example 3 – Solution Common denominator Simplify cont’d
- 56. Evaluating Expressions Involving Inverse Trigonometric Functions
- 57. Evaluating Expressions Involving Inverse Trigonometric Functions Expressions involving trigonometric functions and their inverses arise in calculus.
- 58. Example 4 – Evaluating an Expression Involving Inverse Trigonometric Functions Evaluate sin 2θ, where cos θ
- 59. Example 4 – Solution x2 + y2 = r2 (–2)2 + y2 = 52 y =
- 60. Product-Sum Formulas It is possible to write the product sin u cosν as a sum of
- 61. Product-Sum Formulas Dividing by 2 gives the formula sin u cosν = [sin(u +ν) + sin(u
- 62. Product-Sum Formulas The Product-to-Sum Formulas can also be used as Sum-to-Product Formulas. This is possible because
- 63. Product-Sum Formulas The remaining three of the following Sum-to-Product Formulas are obtained in a similar manner.
- 64. Example 5 – Proving an Identity Verify the identity . Solution: We apply the second Sum-to-Product
- 65. Example 5 – Solution Simplify Cancel cont’d
- 66. 7.4 Basic Trigonometric Equations
- 67. Basic Trigonometric Equations An equation that contains trigonometric functions is called a trigonometric equation. For example,
- 68. Basic Trigonometric Equations Solving any trigonometric equation always reduces to solving a basic trigonometric equation—an equation
- 69. Example 1 – Solving a Basic Trigonometric Equation Solve the equation Solution: Find the solutions in
- 70. Example 1 – Solution We see that sin θ = in Quadrants I and II, so
- 71. Example 1 – Solution Figure 2 gives a graphical representation of the solutions. Figure 2 cont’d
- 72. Example 2 – Solving a Basic Trigonometric Equation Solve the equation tan θ = 2. Solution:
- 73. Example 2 – Solution By the definition of tan–1 the solution that we obtained is the
- 74. Example 2 – Solution A graphical representation of the solutions is shown in Figure 6. You
- 75. Basic Trigonometric Equations In the next example we solve trigonometric equations that are algebraically equivalent to
- 76. Example 3 – Solving Trigonometric Equations Find all solutions of the equation. (a) 2 sin θ
- 77. Example 3 – Solution This last equation is the same as that in Example 1. The
- 78. Example 3 – Solution Because tangent has period π, we first find the solutions in any
- 79. Solving Trigonometric Equations by Factoring
- 80. Solving Trigonometric Equations by Factoring Factoring is one of the most useful techniques for solving equations,
- 81. Example 4 – A Trigonometric Equation of Quadratic Type Solve the equation 2 cos2 θ –
- 82. Example 4 – Solution Because cosine has period 2π, we first find the solutions in the
- 83. Example 4 – Solution The second equation has no solution because cos θ is never greater
- 84. Example 5 – Solving a Trigonometric Equation by Factoring Solve the equation 5 sin θ cos
- 85. Example 5 – Solution Because sine and cosine have period 2π, we first find the solutions
- 86. Example 5 – Solution θ ≈ –0.93 So the solutions in an interval of length 2π
- 87. Example 5 – Solution We get all the solutions of the equation by adding integer multiples
- 88. 7.5 More Trigonometric Equations
- 89. More Trigonometric Equations In this section we solve trigonometric equations by first using identities to simplify
- 90. Solving Trigonometric Equations by Using Identities
- 91. Solving Trigonometric Equations by Using Identities In the next example we use trigonometric identities to express
- 92. Example 1 – Using a Trigonometric Identity Solve the equation 1 + sinθ = 2 cos2θ.
- 93. Example 1 – Solution 2 sinθ – 1 = 0 or sinθ + 1 = 0
- 94. Example 1 – Solution Thus the solutions are θ = + 2kπ θ = + 2kπ
- 95. Example 2 – Squaring and Using an Identity Solve the equation cosθ + 1 = sinθ
- 96. Example 2 – Solution 2 cos θ (cos θ + 1) = 0 2 cosθ =
- 97. Example 2 – Solution Check Your Answers: θ = θ = θ = π cos +
- 98. Example 3 – Finding Intersection Points Find the values of x for which the graphs of
- 99. Example 3 – Solution Using or the intersect command on the graphing calculator, we see that
- 100. Example 3 – Solution Solution 2: Algebraic To find the exact solution, we set f (x)
- 101. Example 3 – Solution The only solution of this equation in the interval (–π /2, π
- 102. Equations with Trigonometric Functions of Multiples of Angles When solving trigonometric equations that involve functions of
- 103. Example 4 – A Trigonometric Equation Involving a Multiple of an Angle Consider the equation 2
- 104. Example 4 – Solution 3θ = Solve for 3θ in the interval [0, 2π) (see Figure
- 105. Example 4 – Solution To get all solutions, we add integer multiples of 2π to these
- 106. Example 4 – Solution (b) The solutions from part (a) that are in the interval [0,
- 107. Example 5 – A Trigonometric Equation Involving a Half Angle Consider the equation (a) Find all
- 108. Example 5 – Solution Since tangent has period π, to get all solutions, we add integer
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