Question

Prove that cos^3x+3cos2x= 4 (cos^6 x-sin^6x)

Answer 1

But this isn't true... If
`x=\frac\pi2`, you have


`\cos^3\frac\pi2+3\cos\pi=0-3=-3`

`4\left(\cos^6\frac\pi2-\sin^6\frac\pi2\right)=4(0-1)=-4`

but of course
`-3\\eq-4`.