Помогите, срочно!4*sin10*sin 50*sin70=

Question

Помогите, срочно!
4*sin10*sin 50*sin70=

Answer 1

4*sin10*sin 50*sin70=4sin(90-80)sin(90-40)sin(90-20)=4cos80cos40cos20=4cos80cos40cos20sin20/sin20=2cos80cos40sin40/sin20=sin160/2sin20=sin(180-20)/2sin20=sin20/2sin20=1/2