cos72度—cos36度=?,要过程.
问题描述：
cos72度—cos36度=?,要过程.
答
cos72—cos=cos(54+18)-cos(54-18)=[cos54cos18-sin54sin18]-cos54cos18+sin54sin18=-2sin54sin18=-2cos36sin18 =-2cos36sin18cos18/cos18 =-cos36sin36/cos18 =-sin72/(2cos18) =-sin72/(2sin72)=-1/2