Có bao nhiêu số 

Câu hỏi: Có bao nhiêu số $a\in (0;20\pi )$ sao cho ${\int }_0^{a}si{n}^{5}xsin2xdx=\frac{2}{7}.$

A. $I=-\left(x-1\right)\cos 2x{|}_0^{\frac{\mathrm{\pi }}{4}}$$+{\int }_0^{\frac{\mathrm{\pi }}{0}}\cos 2xdx$

B. $I=-\left(x-1\right)\cos 2x$$-{\int }_0^{\frac{\mathrm{\pi }}{0}}\cos 2xdx$

C. $I=-\frac{1}{2}\left(x-1\right)\cos 2x{|}_0^{\frac{\mathrm{\pi }}{4}}$$+\frac{1}{2}{\int }_0^{\frac{\mathrm{\pi }}{0}}\cos 2xdx$

D. $I=-\frac{1}{2}\left(x-1\right)\cos 2x{|}_0^{\frac{\mathrm{\pi }}{4}}$$-\frac{1}{2}{\int }_0^{\frac{\mathrm{\pi }}{0}}\cos 2xdx$