how would I justify these identity::confused: (show all the work... )

sin4x=8cos^3xsinx-4


also...

tanx(1-sin^2x)-1/2sin2x


thanks!!

I can give you a good start on #1. Use the addition formula for sine.

Then use sin(2x)=2sin(x)cos(x). Then use cos(2x)=2cos^{2}(x)-1

Then you're all set.:)