f(x) = E(0, inf) [((-1)^n)(x^2n)]/(2n+1)!

And I found f'(x) to be E (0,inf) [2n((-1)^n)(x^(2n-1))]/(2n+1)!

The question is:

Show that y = f(x) is a solution to the differention equation xy'+y=cosx

Not sure really where to start or how to show this. Help appreciated.