Tìm k .

\(T = 2\sin \left( {\frac{{9\pi }}{2} - x} \right) + 3\cos \left( {19\pi - x} \right)\)\( = 2\sin \left( {4\pi + \frac{\pi }{2} - x} \right) + 3\cos \left( {18\pi + \pi - x} \right)\)

\( = 2\sin \left( {\frac{\pi }{2} - x} \right) + 3\cos \left( {\pi - x} \right)\)\( = 2\cos x - 3\cos x = - \cos x\) \( \Rightarrow k = - 1\).

Trả lời: −1.