## nickersia 6 months ago Use De Moivre's Theorem to prove the identity:

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1. nickersia

$\cos 3\alpha=4\cos ^{3}\alpha-3\cos \alpha$

2. nickersia

Anyone?

3. BSwan

simply De Moivre's Theorem :- $$(\cos x+ i \sin x)^n=\cos (nx ) +i \sin(nx)$$