Math Help - A quick function question

1. A quick function question

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

My attempt:
$f(x + 17\pi) = x + 17\pi + cos(2(x+17\pi))$

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

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

2. Originally Posted by Glitch
The question:
Suppose that $f(x) = x + cos(2x)$, write down $f(x + 17\pi)$

My attempt:
$f(x + 17\pi) = x + 17\pi + cos(2(x+17\pi))$

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

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

$\cos(A+B)=\cos(A)\cos(B)-\sin(A)\sin(B)$

CB

3. Thanks dude, you're a champion!