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Trả lời:
$\begin{array}{l}M=\dfrac{\sin3x-\sin x}{2\cos^2x-1}\\\,\,\,\,\,\,\,\,=\dfrac{2.\cos\dfrac{3x+x}{2}.\sin\dfrac{3x-x}{2}}{\cos2x}\\\,\,\,\,\,\,\,\,=\dfrac{2\cos2x.\sin x}{\cos2x}\\\,\,\,\,\,\,\,\,=2\sin x\end{array}$