Question

can someone please help me with this:
Differentiate 2cos7x*cosx when x is pi/12

Answer 1

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Recall:
Product rule:
\[\frac{d}{dx}[f(x)g(x)] = f'(x)g(x)+g'(x)f(x)\]
and
\[\frac{d}{dx}cos(ax) = -asin(ax)\]
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\[\frac{d}{dx}(2cos7x*cosx)\]\[=2(cosx\frac{d}{dx}(cos7x)+cos(7x)\frac{d}{dx}(cosx))\]
Can you continue from here?