Question

cos2x = 1 or more of these are possible.
a. 1-2sin^2 x
b. 2sin^2 x -1
c. sin^2 x - cos^2 x
d. cos^2 x - sin^2 x

Answer 1

a. 1-2sin^2 x

d. cos^2 x - sin^2 x

Answer 2

and this is true why?