Question

How do you verify the identity `cos 4x + cos 2x = 2 - 2 sin^2 2x - 2 sin^2 x`?

Answer 1

Use the trig identity: `cos 2a = 1 - 2.sin^2 a`
`cos 4x = 1 - 2.sin^2 (2x)`
`cos 2x = 1 - 2sin^2 (x)`

`y = cos 4x + cos 2x = 2 - 2.sin^2 (2x) - 2.sin^2 (x)`.