I have tried for quite a while, but am completely stuck now.
Can anyone have ago?
The method that pickslides used was very neat, Tweety.
He used the same identity at each step.
You may see the identity written as
Rearranging this, we get
He simply used this, by letting A=2x to get cos4x
and the powers of x follow.
I started with the powers and worked back, eventually using that identity,
but i also used
The other identity i used is