Найти значение выражения sin20*sin50*sin70/sin80

Решите задачу:

$\frac{sin20*sin50*sin70}{sin80} = \frac{(2cos20*sin20)*sin50*sin70}{2cos20*sin80} = \frac{sin40*sin(90-40)*sin70}{2cos20*sin80} =\\\\= \frac{sin40*cos40*sin70}{2cos20*sin80} = \frac{\frac{1}{2}sin80*sin70}{2cos20*sin80} =\frac{sin(90-20)}{4cos20}=\frac{cos20}{4cos20}=\frac{1}{4}\\\\P.S.\; \; \; sin(90- \alpha )=cos \alpha \; \; ;\; \; \; cos(90- \alpha )=sin \alpha$