evaluate f(x) = xcosx?

Question

under advance mathematics

Answer 1

Is it f `(x) that is required?
f `(x) = cos x - (sin x)(x)
f `(x) = cos x - x sin x

Answer 2

Since a series expansion of cos x provides,

cos x = 1 - x^2/2! + x^4/4! - x^6/6! ...,

x cos x = x - x^3/2! + x^5/4! - x^7/6! ...

∞
∑ (-1)^[(n-1)/2] x^n / (n-1)!
n=1,3,5...

What else is needed to "evaluate" f(x)??