If f(x+y) + f(x-y) =2f(x)f(y) and f(0)=k then find f(x)?

[succhi]

if
f(x+y) + f(x-y) =2f(x)f(y)
and f(0)=k
then find f(x)?

What should be approach followed to solve this question .
please ans it soon

 

[CompuChip]

You have an expression with f(x) and f(y) and you only want f(x). So you have to get rid of y.

One more hint: that identity holds for all values of y.