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Verified answer
Cos 6x = cos (3x + 3x)= cos^2 3x - sin^2 3x
= cos^2 3x - (1 - cos^2 3x)
= 2 cos^2 3x - 1
cos 6x = cos^2 3x - sin^2 3x
= (1 - sin^2 3x ) - sin^2 3x
= 1 -2 sin^2 3x