Double angle Cos 2A

Question

Find the exact value of cos2A

given sinA = -7/25

in quadrant IV

Answer 1

There are a few double-angle cosine formulas, but the one that's most useful here is cos2A = 1 - 2(sinA)².

So cos2A in this case is 1 - 2(-7/25)², which is 527/625.