cos (180/5)* cos(360/5)=?
问题描述:
cos (180/5)* cos(360/5)=?
答
cos (180/5)* cos(360/5)
=cos 36* cos72
=cos 36*sin 36*cos72/sin 36
=1/2sin72*cos72/sin 36
=1/4sin144/sin 36
=1/4sin36/sin 36
=1/4