Differentiation
Derivative of inverse functions
GRB_1000_SCQ
Grade Class 12
Question:
Let g(x) = 1/f⁻¹(x). Given the following data:
x: 0, 1, 2, 3, 4
f(x): -2, -1, 2, 4, 6
f'(x): 1/2, 2/3, 1, 4/3, 5/3
The value of g'(4) is:
Step-by-Step Solution
Key Concept: Derivative of inverse function, chain rule, composite function differentiation
Step 1:
Recognize the relationship between $g(x)$ and $f^{-1}(x)$.
We are given that $g(x) = \frac{1}{f^{-1}(x)}$, which can be rewritten as:
$$g(x) = [f^{-1}(x)]^{-1}$$
Step 2:
Differentiate $g(x)$ using the chain rule.
Applying the chain rule to $g(x) = [f^{-1}(x)]^{-1}$:
$$g'(x) = -1 \cdot [f^{-1}(x)]^{-2} \cdot \frac{d}{dx}[f^{-1}(x)]$$
$$g'(x) = -\frac{1}{[f^{-1}(x)]^2} \cdot \frac{d}{dx}[f^{-1}(x)]$$
Step 3:
Apply the inverse function derivative formula.
The derivative of an inverse function is given by:
$$\frac{d}{dx}[f^{-1}(x)] = \frac{1}{f'(f^{-1}(x))}$$
Step 4:
Substitute the inverse function derivative formula into the expression for $g'(x)$.
$$g'(x) = -\frac{1}{[f^{-1}(x)]^2} \cdot \frac{1}{f'(f^{-1}(x))}$$
Step 5:
Find $f^{-1}(4)$ using the given data table.
Looking at the table, we need to find the value of $x$ such that $f(x) = 4$.
From the data: $f(3) = 4$
Therefore: $f^{-1}(4) = 3$
Step 6:
Find $f'(f^{-1}(4))$ using the data table.
Since $f^{-1}(4) = 3$, we need $f'(3)$.
From the table: $f'(3) = \frac{4}{3}$
Step 7:
Calculate $g'(4)$ by substituting all values.
$$g'(4) = -\frac{1}{[f^{-1}(4)]^2} \cdot \frac{1}{f'(f^{-1}(4))}$$
$$g'(4) = -\frac{1}{(3)^2} \cdot \frac{1}{\frac{4}{3}}$$
$$g'(4) = -\frac{1}{9} \cdot \frac{3}{4}$$
$$g'(4) = -\frac{3}{36} = -\frac{1}{12}$$
**Final Answer:** The value of $g'(4) = -\frac{1}{12}$, which corresponds to **Option 1**.
Correct Answer: 3