sin10°*sin30°*sin50°*sin70°
= sin10°*sin30°*cos40°*cos20°
= sin10°*cos20°*cos40°/2
= sin10°*cos10°*cos20°*cos40°/(2 cos10°)
=1/16
既然sin10°*sin30°*sin50°*sin70°=1/16
则：sin10°sin30°sin50°sin80°≠1/16
sin10°sin30°sin50°sin80°
= sin10°cos10°sin50°/2
=sin20°cos40°/2
= sin20° (1-2 sin²20°)/2
要化简：必须解一元三次方程,利用cos3a与cosa的关系.