cos72度减cos26度等于多少,求详解.
问题描述：
cos72度减cos26度等于多少,求详解.
错了是cos72-cos36
答
cos72-cos36
=cos(54+18)-cos(54-18)
=(cos54cos18-sin54sin18)-(cos54cos18+sin54sin18)
=-2sin54sin18
=-2cos36sin18
=-2cos36sin18cos18/cos18
=-cos36sin36/cos18
=-sin72/(2cos18)
=-sin72/(2sin72)=-1/2