cos (180/5)* cos(360/5)=?
问题描述:
cos (180/5)* cos(360/5)=?
答
原式=cos36*cos72=cos36*(2*cos36*cos36-1)=2*cos36*cos36*cos36-cos36
cos36=cos0.2π=0.8090169943749474 下面的你自己会算了吧?给分吧 呵呵
答
1/4
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