2010-10-02, 08:37 PM
Russt Wrote:Wait, what? No you can't. You can use difference of squares, though.
cos^4 - sin^4 = (cos^2)^2 - (sin^2)^2 = (cos^2 + sin^2)(cos^2 - sin^2) = 1(cos^2 - sin^2) = cos 2x.
arccos cos 2x = 2x.
Just have to be a bit careful with arccos ( cos 2x). Arccos maxes out at pi, where as 2x doesn't. So if you plug in pi to both sides of the equations and work them out the long way, you wont get the same answer.
Notedly you get 0 = 2*PI