Solution
Expression : cos3A = X
= $$cos(2A+A)$$
= $$cos(2A)cosA-sin(2A)sinA$$
= $$(2cos^2A-1)cosA - (2sinAcosA)sinA$$
= $$2cos^3A-cosA - (sin^2A)2cosA$$
= $$2cos^3A-cosA- 2cosA(1-cos^2A)$$
= $$2cos^3A-cosA-2cosA+2cos^3A$$
= $$4cos^3A-3cosA$$
=> Ans - (A)
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