化简cos20° cos140° cos260°=---急用!

来源：学生作业学帮网编辑：学帮网时间：2024/06/05 19:23:47

化简cos20° cos140° cos260°=---
急用!

=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