Answer
$$ \displaystyle\int 2{\cdot}\cos\left(x\right){\cdot}x{\cdot}\cos\left(3\right){\cdot}x\, \mathrm d x = 2\,{\it cos}_{3}\,\left(x^2\,\sin x-2\,\sin x+2\,x\,\cos x\right) $$
Explanation
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