求问:cos20×cos40×cos80=?要求有具体过程.
问题描述:
求问:cos20×cos40×cos80=?要求有具体过程.
答
cos20°cos40°cos80°
=2sin20°cos20°cos40°cos80°/2sin20°
=sin40°cos40°cos80°/2sin20°
=sin80°cos80°/4sin20°
=sin160°/8sin20°
=sin20°/8sin20°
=1/8