Trig identities?

Homework Statement

How to get from

$$\sqrt{2+2cosx}$$
to

2cos(x/2)

The Attempt at a Solution

I'm pretty much stuck on this.
I can only see that is simplifies by taking out the common factor 2, which isnt correct.

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rock.freak667
Homework Helper
Well id cos2x=2cos2x-1, then cosx=2cos2(x/2)-1

Yup, thanks!