Posted by Mitchel on Sunday, February 17, 2013 at 11:33pm.
using the identity
cos 2A = cos^2 A - sin^2 A = 1 - 2sin^2 x = 2cos^2 x -1
LS = cos 4x + cos 2x
= cos^2 2x - sin^2 2x + cos 2x
= (1 - sin^2 (2x) ) - sin^2 (2x) + (1 - 2sin^2 x)
= 1 - 2sin^2 (2x) + 1 - 2sin^2 x
= 2 - 2sin^2 (2x) - 2sin^2 x
= RS
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