Thread: Identity proof

Hello,

I am having trouble simplifying this identity [cos4x= 2cos^2 2x-1]. I thought I could solve it with the double angle identity, but the 2x seems to be getting in the way. Can I remove the 2x and multiply it by the identity cos2x?

Originally Posted by Sedgewick19

Hello,

I am having trouble proving this identity [cos4x= 2cos^2 2x-1]. I thought I could solve it with the double angle identity, but the 2x seems to be getting in the way. Can I remove the 2x and multiply it by the identity cos2x?

It's just a simple substitution really...

you know.

Well in the case, just set .

Then you have

Thanks, I thought that may have been the way the solve the problem, but I was afraid that it was too easy.