10°的正弦*50°的正弦*70°的正弦=?
sin10°sin50°sin70°
= 2sin10°cos10°sin50°cos20°/2cos10°
= 2sin20°cos20°sin50°/4cos10°
= 2sin40°cos40°/8cos10°
= sin80°/16cos10°
= cos10°/16cos10°
=1/16