化简cos20° cos140° cos260°=---急用!
问题描述：
化简cos20° cos140° cos260°=---
急用!
答
=cos20*(-cos40)*(-cos80)
=sin20*cos20*(-cos40)*(-cos80)/sin20
=1/2*sin40*(-cos40)*(-cos80)/sin20
=-1/4*(sin80)*(-cos80)/sin20
=1/8*(sin160)/sin20
=1/8