How do find the derivative of # y = cos^2(x)#?

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Answer 1 · 2015-09-15T02:55:46

First of all #y=cos^2x=(cosx)^2#

Hence

#y'=2cosx*(cosx)'=2cosx*(-sinx)=-2cosx*sinx=-sin2x#

Another way is

#y=cos^2x=1/2(1+cos2x)#

Hence

#y'=1/2*(-sin2x *(2x)')=-sin2x#