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QUESTION
How do you prove ##cos2x = 2cos^2x -1##?
Use the identity: cos (a + b) = cos a.cos b - sin a.sin b
##cos 2x = cos (x + x) = cos x.cos x - sin x. sin x = cos^2 x - sin^2 x ##=
##= cos^2 x - (1 - cos^2 x) = 2cos ^2 x - 1 ##
