Double angle cos(2a)

Question

Given  and  is in quadrant III. Use double angle formulas to find the following:

Answer 1

cos 2x = cos²x - sin²x = 1 - 2sin²x and sin x = sqrt[(1-cos 2x)/2]

You can do a similar manipulation to get cos x.

You should check yourself, by actually getting 2x and get the functions directly.