Use a double-angle formula to find the exact value of cos2x when sinx = 3/5, where pi/2 < x < pi.

Question

-(7/25) or 7/25?

Answer 1

well, just keep track of the signs as you work it out.

cos2x = 1 - 2sin^2x = 1 - 2*9/25 = 7/25

Or, if you insist on doing it the hard way,

cos2x = cos^2x - sin^2x = 16/25 - 9/25 = 7/25

If you came up with -7/25, you need to remember your simple Algebra I!