The question:

Suppose that $\displaystyle f(x) = x + cos(2x)$, write down $\displaystyle f(x + 17\pi)$

My attempt:

$\displaystyle f(x + 17\pi) = x + 17\pi + cos(2(x+17\pi))$

But the answer is, $\displaystyle x + 17\pi + cos(2x)$

Why don't we substitute the x inside the cosine function? Thanks!