Tìm 

Câu hỏi: Tìm $I = \int \sin 5x\cos xdx .$

A. $I = -\frac{1}{5}\cos 5x +C$

B. $I = \frac{1}{5}\cos 5x +C$

C. $I = -\frac{1}{8}\cos 4x -\frac{1}{12}\cos 6x+C$

D. $I = \frac{1}{8}\cos 4x +\frac{1}{12}\cos 6x+C$