cos20°cos40°cos80°=（2sin20°cos20°cos40°cos80°)/2sin20°
=（sin40°cos40°cos80°)/2sin20°
=(2sin40°cos40°cos80°)/4sin20°
=(sin80°cos80°)/4sin20°
=(2sin80°cos80°)/8sin20°
=sin160°/8sin20°
=sin20°/8sin20°(sin(180°-a)=sina)
=1/8
求cos20°cos40°cos80°的值
cos20°cos40°cos80°=（2sin20°cos20°cos40°cos80°)/2sin20°
=（sin40°cos40°cos80°)/2sin20°
=(2sin40°cos40°cos80°)/4sin20°
=(sin80°cos80°)/4sin20°
=(2sin80°cos80°)/8sin20°
=sin160°/8sin20°
=sin20°/8sin20°(sin(180°-a)=sina)
=1/8