Алгебра: (2cos^x-5cosx+2)*sqrt(-7sinx)=0 реши,любить буду!:)

Решите задачу:

$(2cos^{2}x-5cosx+2)\cdot \sqrt{-7sinx}=0\\\left \{ {{2cos^{2}x-5cosx+2=0} \atop {sinx=0}} \right.\\cosx=t\\\left \{ {{2t^{2}-5t+2=0} \atop {sinx=0}} \right.\\D=25-16=9\\t_{1}=2,\ t_{2}=\frac{1}{2}\\ \left \{ {{cosx=\frac{1}{2} } \atop {sin x=0}} \right. \\ x=\pm \frac{\pi}{3} +2\pi n, n \in Z ,\\ x= \pi k, k \in Z$