cos6 cos42 cos66 cos78
问题描述：
cos6 cos42 cos66 cos78
cos6*cos54*cos66=1/4 * cos18
why? thanks
答
解 据已知恒等式:cosx*cos(60°+x)*cos(60°-x)=cos(3x)/4 cos6°*cos42°*cos66°*cos78°= cos6°*cos66°*[ cos18°*cos42°*cos78°/ cos18°] = cos6°*cos66°*cos54°/(4 cos18°) = cos18°/(16 cos18°)=1/...