Tính a + b + c + d .

Ta có \(4\sin 3x\sin 2x\cos x = 4\left( {\sin 3x\cos x} \right)\sin 2x = 4 \cdot \frac{1}{2}\left( {\sin 4x + \sin 2x} \right)\sin 2x\)

            \( = 2\sin 4x\sin 2x + 2{\sin ^2}2x = \left( {\cos 2x - \cos 6x} \right) + 2\left( {\frac{{1 - \cos 4x}}{2}} \right)\)\( = \cos 2x - \cos 4x - \cos 6x + 1\).

Vậy \(a + b + c + d = 0\).

Đáp án:0.