Question

Someone please help with deriving cosine function, I don't even know where to start. Question attached

Answer 1

\[\huge \cos4\theta = \cos 2(2\theta) =2 \cos^2(2 \theta)-1\]

Answer 2

or
\[\huge \cos4\theta = \cos 2(2\theta) =1-2 \sin^2(2 \theta)\]

Answer 3

or
\[\huge \cos4\theta = \cos 2(2\theta) =2[\cos^2 (2\theta) - \sin^2(2 \theta)]\]

Answer 4

@Azureilai

Answer 5

ok I see. So expand it and then simplify?