Section 7.5 Inverse Trigonometric Functions II

Transcription

1 Section 7.5 Inverse Trigonometric Functions II Note: A calculator is helpful on some exercises. Bring one to class for this lecture. OBJECTIVE : Evaluating composite Functions involving Inverse Trigonometric Funcitons of the Form f! f and f! f It is imperative that you know and understand the three inverse trigonometric functions introduced in 7.4. A. y = sin x (Say: y is the angle whose sine is x ). Draw the graph of the inverse sine function. 3. The range of the inverse sine function represents an angle whose terminal side lies in B. y = cos x (Say: y is the angle whose cosine is x ). Draw the graph of the inverse cosine function. 3. The range of the inverse cosine function represents an angle whose terminal side lies in C. y = tan x (Say: y is the angle whose tangent is x ). Draw the graph of the inverse tangent function. 3. The range of the inverse tangent function represents an angle whose terminal side lies in

2 CAUTION: For trigonometric expressions of the form ( f! f )(x) or ( f! f )(x), the cancellation equations work ONLY if x is in the domain of the inner function. Cancellation Equations for Compositions of Inverse Trigonometric Functions Cancellation Equations for the Restricted Sine Function and its Inverse sin sin = sin ( sin ) x x for all x in the interval [,] π π θ = θ for all θ in the interval, 2 2. Cancellation Equations for the Restricted Cosine Function and its Inverse cos cos = cos x x for all x in the interval [,] ( cosθ) = θ for all θ in the interval [ ] 0,π. Cancellation Equations for the Restricted Tangent Function and its Inverse tan tan = tan ( tan ) x x for all x in the interval (, ). θ = θ for all θ in the interval ( π, π ). EXAMPLES: Find the exact value of each expression or state that it does not exist sin sin 3 %% $ # # 2 ' cos cos 8 %% $ ' # # 5& & sin sin π % # 7 & cos $ cos 3π # 4 % ' & tan tan 7π % # 6 &

3 7.5.7 tan tan 8π % # 3 & cos cos 3π % # 0 & OBJECTIVE 2: Evaluating composite Functions involving Inverse Trigonometric Funcitons of the Form f! g and f! g Method:. Evaluate the inner expression and then evaluate the outer expression. 2. It may be necessary to draw a triangle (using x, y, or r) in the appropriate quadrant (depending on if the trig value is positive or negative), determine the value of the missing side and write the trig function requested in the outer expression. 3. If an exact value of the inner expressions cannot be determined, try writing the expressions as an equivalent expression using a cofunction identity. EXAMPLES. : Find the exact value of each expression or state that it does not exist cos( tan ) tan sin %% $ ' # # tan sin 3 %% $ # # 4 ' cos $ sin$ 5π # # 4 %% '' cos sin 9π %% $ ' # # 9

4 OBJECTIVE 3: Functions Understanding the Inverse cosecant, Inverse Secant, and Inverse Cotangent Inverse Cosecant Function The inverse cosecant function, denoted as y =esc-' x, is the inverse of y = cscx, [- f,o)u(o,f ] The domain of y = csc- x is( - oo,-l]u[l, oo ) and the range is [- -f, O )u ( O,f]. y nf I y = esc x X Inverse Secant Function The inverse secant function, denoted as y = sec- x, is the inverse of y = secx, [O,f) U ( f,n J. ( -, J[) y ) - I y = sec x The domain of y =sec-' x is( - oo, -l]u[l,oo )and the range is 2 0 (, 0) )' Inverse Cotangent Function X The inverse cotangent function, denoted as y = COC X, is the inverse of. y = COt X,. (- ~ 0 ) \){ 0 I % J The domain of y = coc xis ( - oo,oo )and the range is (-f,o)u ( O,f J. EXAMPLES. : Find the exact value of each expression or state that it does not exist csc- (2)

5 cot % # 3 & OBJECTIVE 4: Writing Trigonometric Expressions as Algebraic Expressions Functions In Calculus, it is often necessary to write trigonometric expressions algebraically. In this text u is used as the unknown variable. In calculus x is often (but not always) used. We assume that the variable represents an angle whose terminal side is located in Quadrant I Method:. Given the inverse trigonometric expression (inner expression which represents an unknown angle θ ), draw the triangle represented with θ in standard position and the terminal side located in QI 2. Label the given sides of the triangle. Since trigonometric expressions represent ratios of the sides of right triangles, two sides are always given. 3. Determine algebraically the 3 rd side. 4. Write the expression asked for (outside expression). EXAMPLES. Rewrite each trigonometric expression as an algebraic expression involving the variable u. Assume that u > 0 and that the value of the inner trigonometric expression represents an angle θ such that 0 < θ < π tan(cos 2u) cos sin 3% # u& sec sin u % $ ' # u 2 +2 &