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use the identity for cos(2y) with y=2x.
please elaborate, I dont think my trig book explained double angles beyond the basic identities, all I can think of doing is multiplying double id's together but I dont see how you double a double angle, that is to say make 2x=4x.
cos(4x)=2cos^2(2x)-1 comes from cos(2x)=2cos^2(x)-1 when substituting 2x for x in the later equation.
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