求值 cos10°cos20°cos40°
问题描述:
求值 cos10°cos20°cos40°
答
cos10°cos20°cos40°=2sin10° cos10°cos20°cos40°/2sin10°
=2sin20°cos20°cos40°/4sin10° =2sin40°cos40°/8sin10° =sin80°/8sin10° =tan80°/8