Question 1072510
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prove the following identity

1. cos^2 3x - cos6x = sin^2 3x
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Apply the formula cos(2a) = cos^2(a) - sin^2(a).

Take a = 3x. You will get

cos(6x) = cos^2(3x) - sin^2(3x), 

which is what has to be proved.
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