Question

how to solve this question

Answer 1

First recall that cos3(2x)=(cos(2x))3

So,

ddx(cos3(2x))=3cos2(2x)ddx(cos(2x))

=3cos2(2x)(−sin(2x))ddx(2x)

=3cos2(2x)(−sin(2x))(2)

=−6cos2(2x)sin(2x)

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