Question

\[\cos 5 \theta = 16\cos^5 \theta - 20 \cos^3 \theta + 5 \cos \theta\]

How can I prove this through de Moivre theorem?
I could probably force my way through it with Double angle formulae but apparently this is simpler...

Answer 1

oh srry , ok with moivre: , so we need to add some i*sin(theta) if you want to use that