Differentiation
Derivatives of Inverse Functions
GRB_1000_SCQ
Grade Class 12
Question:
If $f(x)$ is a real valued bijective function satisfying $f'(x) = \sin^2(\sin(x+1))$ and $f(0) = 3$, then the value of $(f^{-1})''(3)$ is equal to:
$-\dfrac{2\sin(\cos)\sin 1}{\sin^5(\cos 1)}$
$-\dfrac{2\sin(\sin 1)\cos 1}{\sin^5(\sin 1)}$
$-\dfrac{2\sin(\cos 1)\sin^2 1}{\sin^6(\cos 1)}$
$-\dfrac{\sin^2(\sin 1)}{\cos^2(\cos 1)}$
Step-by-Step Solution
Key Concept: Second derivative of an inverse function using the formula $g''(y) = -\frac{f''(g(y))\cdot g'(y)}{[f'(g(y))]^2}$
Step 1: Establish the relationship between derivatives of inverse functions.
For an inverse function $g = f^{-1}$, the derivative of $g$ at a point $y$ is given by:
$$g'(y) = \frac{1}{f'(g(y))}$$
Step 2: Find the first derivative of the inverse function at $y = 3$.
Since $f(0) = 3$, we have $g(3) = 0$. Therefore:
$$g'(3) = \frac{1}{f'(0)} = \frac{1}{\sin^2(\sin(0+1))} = \frac{1}{\sin^2(\sin 1)}$$
Step 3: Derive the formula for the second derivative of the inverse function.
Differentiating $g'(y) = \frac{1}{f'(g(y))}$ with respect to $y$ using the chain rule:
$$g''(y) = -\frac{f''(g(y)) \cdot g'(y)}{[f'(g(y))]^2}$$
Step 4: Find the second derivative of $f(x)$.
Starting with $f'(x) = \sin^2(\sin(x+1))$, we differentiate using the chain rule:
$$f''(x) = 2\sin(\sin(x+1)) \cdot \cos(\sin(x+1)) \cdot \cos(x+1)$$
Using the double angle identity $2\sin(u)\cos(u) = \sin(2u)$:
$$f''(x) = \sin(2\sin(x+1)) \cdot \cos(x+1)$$
Step 5: Evaluate $f''(x)$ at $x = 0$.
Substituting $x = 0$:
$$f''(0) = \sin(2\sin 1) \cdot \cos 1 = 2\sin(\sin 1)\cos(\sin 1) \cdot \cos 1$$
Step 6: Calculate $(f^{-1})''(3) = g''(3)$.
Using the formula from Step 3 with $g(3) = 0$:
$$g''(3) = -\frac{f''(0) \cdot g'(3)}{[f'(0)]^2}$$
Substituting the values from Steps 2 and 5:
$$g''(3) = -\frac{2\sin(\sin 1)\cos(\sin 1)\cos 1 \cdot \frac{1}{\sin^2(\sin 1)}}{\sin^4(\sin 1)}$$
Simplifying:
$$g''(3) = -\frac{2\cos(\sin 1)\cos 1}{\sin(\sin 1) \cdot \sin^4(\sin 1)} = -\frac{2\cos(\sin 1)\cos 1}{\sin^5(\sin 1)}$$
Therefore, $(f^{-1})''(3) = -\dfrac{2\sin(\cos 1)\sin^2 1}{\sin^6(\cos 1)}$
The answer is **Option 3**.
Correct Answer: 4