Functional Equation

If f(x) = f(xy)+f(\frac{x}{y}) \text{for all } x \epsilon \mathbb{R^{+}} and f(1) = 0 , f ' (1) = 0 , then find the f(x).

Please HelpI just can't make this : f ' (1) = 0 ?

5 Answers

262
Aditya Bhutra ·

any condition for y ?
or else putting y=1 in the eqn ,
we get f(x)=0

71
Vivek @ Born this Way ·

No not given i presume. But certainly f(x) = 0 won't satisfy the given criteria, so it isn't a solution. Try finding other.

1
rahulsidhu ·

@Vivek- which of the criteria would f(x) =0 not satisfy?

1
jee12 ·

@Vivek- how is it that f(x)=0 won't satisfy??

71
Vivek @ Born this Way ·

Hmm.. Ok. That's the trivial one and probably the solution for the problem as well. Cause I see no other function goes to the second criteria i.e., f'(1)=0

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