400
For this trig identity, you pick which alternate form to use based on what you already know or what else you have.
What is cos 2x?
Ex.: sin x = 3/5. Find cos 2x:
cos 2x = 1 - 2sin2 x
cos 2x = 1 - 2s2
cos 2x = 1 - (3/5)2
cos 2x = 1 - 9/25
cos 2x = 16/25