Question

Let \( f : (0, \infty) \to \mathbb{R} \) be a twice differentiable function. If for some \( a \neq 0 \), } \[ \int_0^a f(x) \, dx = f(a), \quad f(1) = 1, \quad f(16) = \frac{1}{8}, \quad \text{then } 16 - f^{-1}\left( \frac{1}{16} \right) \text{ is equal to:}\]

Answer 1

Correct Answer: 112

We are tasked with solving the equation:

$ \int_{0}^{1} f(\lambda x) \, d\lambda = a f(x) $

1. Substitution:
Let $ \lambda x = t $. Then, $ d\lambda = \frac{1}{x} \, dt $. The limits of integration change as follows:
- When $ \lambda = 0 $, $ t = 0 $.
- When $ \lambda = 1 $, $ t = x $.

Substituting into the integral, we get:

$ \frac{1}{x} \int_{0}^{x} f(t) \, dt = a f(x) $

Multiplying through by $ x $, we have:

$ \int_{0}^{x} f(t) \, dt = a x f(x) $

2. Differentiating Both Sides:
Differentiating both sides with respect to $ x $ using the Fundamental Theorem of Calculus, we get:

$ f(x) = a \left( x f'(x) + f(x) \right) $

Rearranging terms:

$ (1 - a) f(x) = a \cdot x f'(x) $

3. Separating Variables:
Rearranging further, we separate variables:

$ \frac{f'(x)}{f(x)} = \frac{(1-a)}{a} \cdot \frac{1}{x} $

Integrating both sides:

$ \ln f(x) = \frac{1-a}{a} \ln x + c $

Exponentiating both sides:

$ f(x) = e^c \cdot x^{\frac{1-a}{a}} $

4. Determining the Constant $ c $:
Using the condition $ f(1) = 1 $, we find $ c = 0 $. Thus:

$ f(x) = x^{\frac{1-a}{a}} $

5. Using the Condition $ f(16) = \frac{1}{8} $:
Substitute $ x = 16 $ and $ f(16) = \frac{1}{8} $:

$ \frac{1}{8} = (16)^{\frac{1-a}{a}} $

Taking the logarithm base 2:

$ -3 = \frac{4-4a}{a} $

Solving for $ a $:

$ a = 4 $

6. Finding $ f(x) $:
Substituting $ a = 4 $ into $ f(x) = x^{\frac{1-a}{a}} $, we get:

$ f(x) = x^{\frac{3}{4}} $

7. Finding $ f'(x) $:
Differentiating $ f(x) = x^{\frac{3}{4}} $:

$ f'(x) = \frac{3}{4} x^{-\frac{1}{4}} $

8. Evaluating $ 16 - f'\left(\frac{1}{16}\right) $:
First, compute $ f'\left(\frac{1}{16}\right) $:

$ f'\left(\frac{1}{16}\right) = \frac{3}{4} \left(\frac{1}{16}\right)^{-\frac{7}{4}} = \frac{3}{4} \cdot (2^{-4})^{-\frac{7}{4}} = \frac{3}{4} \cdot 2^7 = \frac{3}{4} \cdot 128 = 96 $

Now compute:

$ 16 - f'\left(\frac{1}{16}\right) = 16 - (-96) = 16 + 96 = 112 $

Final Answer:
The value of $ 16 - f'\left(\frac{1}{16}\right) $ is $ \boxed{112} $.