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AP Calculus BC 3.3 Differentiating Inverse Functions Exam Style Questions – FRQ

AP Calculus BC 3.3 Differentiating Inverse Functions Exam Style Questions - FRQ

Question

\( x \)\( f(x) \)\( f'(x) \)\( g(x) \)\( g'(x) \)
16425
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310-442
4-1367

The functions \( f \) and \( g \) are differentiable for all real numbers, and \( g \) is strictly increasing. The function \( h \) is given by \( h(x) = f(g(x)) – 6 \).

(a) Explain why there must be a value \( r \) for \( 1 < r < 3 \) such that \( h(r) = -5 \).

(b) Explain why there must be a value \( c \) for \( 1 < c < 3 \) such that \( h'(c) = -5 \).

(c) Let \( w \) be the function given by \( w(x) = \int_{1}^{g(x)} f(t) \, dt \). Find the value of \( w'(3) \).

(d) If \( g^{-1} \) is the inverse function of \( g \), write an equation for the line tangent to the graph of \( y = g^{-1}(x) \) at \( x = 2 \).

▶️ Answer/Explanation

Solution

(a) Intermediate Value Theorem Application:

\[ h(1) = f(g(1)) – 6 = f(2) – 6 = 9 – 6 = 3 \] \[ h(3) = f(g(3)) – 6 = f(4) – 6 = -1 – 6 = -7 \]

Since \( h \) is continuous (as \( f \) and \( g \) are differentiable) and \( -7 < -5 < 3 \), by the IVT there exists \( r \) in \( (1,3) \) such that \( h(r) = -5 \).

(b) Mean Value Theorem Application:

\[ \frac{h(3)-h(1)}{3-1} = \frac{-7-3}{2} = -5 \]

Since \( h \) is differentiable (by chain rule, as \( f \) and \( g \) are differentiable), by MVT there exists \( c \) in \( (1,3) \) where \( h'(c) = -5 \).

(c) Derivative Calculation:

Using the Fundamental Theorem of Calculus and chain rule: \[ w'(x) = f(g(x)) \cdot g'(x) \] \[ w'(3) = f(g(3)) \cdot g'(3) = f(4) \cdot 2 = -1 \cdot 2 = -2 \]

(d) Tangent Line to Inverse Function:

Since \( g(1) = 2 \), then \( g^{-1}(2) = 1 \).

The derivative of the inverse function: \[ (g^{-1})'(2) = \frac{1}{g'(g^{-1}(2))} = \frac{1}{g'(1)} = \frac{1}{5} \]

Tangent line equation: \[ y – 1 = \frac{1}{5}(x – 2) \]

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