For questions which ask for cos nθ = ... or sin nθ = ...
Use de Moivre's Theorem + binomial expansion
Compare real and imaginary parts
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For questions which ask for cos^n θ = ... or sin^n θ = ...
let z = Cos θ +iSin θ
cos^n θ = (z + z^-1)^n
sin^n θ = (z - z^-1)^n
Binomially expand bracket
Group like terms
Replace like terms with 2Cosn θ OR 2iSin nθ
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