Solution
Using triple angle formula, we know that : $$cos(3A) = 4cos^3A - 3cosA$$
= $$cos^3A + (3cos^3A - 3cosA)$$
= $$cos^3A + 3cosA(cos^2A - 1)$$
= $$cos^3A - 3cosA(1 - cos^2A)$$
= $$cos^3A - 3sin^2AcosA$$
=> Ans - (A)
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