What is the derivative of $f (x) = cos x \cdot cos 2 x$ $f(x)=cosx*cos2x$ ?

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Answer 1 · 2015-11-19T16:14:18

$- sin x cos 2 x - 2 sin 2 x cos x$ $-sinxcos2x-2sin2xcosx$

Also, find the derivatives of the two terms first.

$\frac{d}{d x} [cos x] = - sin x$ $d/dx[cosx]=-sinx$
$\frac{d}{d x} [cos 2 x] = - 2 sin 2 x$ $d/dx[cos2x]=-2sin2x$

$\frac{d}{d x} [cos x \cdot cos 2 x] = - sin x cos 2 x - 2 sin 2 x cos x$ $d/dx[cosx*cos2x]=-sinxcos2x-2sin2xcosx$

What is the derivative of f(x)=cosx⋅cos2xf(x)=cosx*cos2x?