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If f:R rarr (0,oo)be a differentiable f...

If `f:R rarr (0,oo)`be a differentiable function `f(x)` satisfying
`f(x+y)-f(x-y)=f(x)*{f(y)-f(-y)}, AA x, y in R, (f(y)!= f(-y) " for all " y in R)` and `f'(0)=2010`.
Now, answer the following questions.
Which of the following is true for `f(x)`

A

`f(x)` is one-one and into

B

`{f(x)}` is non-periodic, where `{*}` denotes fractional part of `x`.

C

`f(x)=4` has only two solutions.

D

`f(x)=f^(-1)(x)` has only one solution .

Text Solution

AI Generated Solution

The correct Answer is:
To solve the problem, we need to analyze the given functional equation and the properties of the function \( f(x) \). Let's go through the steps systematically. ### Step 1: Analyze the Functional Equation The functional equation given is: \[ f(x+y) - f(x-y) = f(x) \cdot (f(y) - f(-y)) \] for all \( x, y \in \mathbb{R} \). ### Step 2: Differentiate the Functional Equation We differentiate both sides of the equation with respect to \( y \) at \( y = 0 \): \[ \frac{d}{dy}[f(x+y) - f(x-y)] \bigg|_{y=0} = \frac{d}{dy}[f(x) \cdot (f(y) - f(-y))] \bigg|_{y=0} \] Using the chain rule on the left side: \[ f'(x+0) + f'(x-0) = f'(x) \cdot (f(0) - f(0)) + f(x) \cdot (f'(0) + f'(0)) \] This simplifies to: \[ 2f'(x) = 2f(x)f'(0) \] ### Step 3: Simplify the Equation Dividing both sides by 2: \[ f'(x) = f(x)f'(0) \] ### Step 4: Substitute the Given Value We know that \( f'(0) = 2010 \): \[ f'(x) = 2010 f(x) \] ### Step 5: Solve the Differential Equation This is a separable differential equation. We can rewrite it as: \[ \frac{f'(x)}{f(x)} = 2010 \] Integrating both sides: \[ \int \frac{1}{f(x)} df = \int 2010 \, dx \] This gives us: \[ \ln |f(x)| = 2010x + C \] Exponentiating both sides: \[ f(x) = e^{2010x + C} = Ce^{2010x} \] where \( C = e^C \) is a constant. ### Step 6: Determine the Constant \( C \) To find \( C \), we use the condition \( f(0) \): \[ f(0) = Ce^{2010 \cdot 0} = C \] Since \( f(x) \) must be positive for all \( x \), we can set \( C = 1 \): \[ f(x) = e^{2010x} \] ### Step 7: Analyze the Properties of \( f(x) \) The function \( f(x) = e^{2010x} \) is an increasing function. The fractional part of an increasing function is non-periodic. ### Conclusion Thus, the correct statement regarding \( f(x) \) is: - The fractional part of \( f(x) \) is non-periodic.
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