# Prove that:

We know that,

cos 3θ =4cos3θ–3cosθ

4 cos3θ=cos3θ+3cosθ

And similarly

sin 3θ=3sin θ–4sin3 θ

4 sin3θ=3sinθ–sin 3θ

Now,

Substituting the values from equation (i) and (ii), we get

(as sin(x+y) = sin x cos y+cos x sin y)

RHS

Hence Proved

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