Question

数学计算求解！！！

cos72°-cos36°=？
不要直接给答案-1/2，我要过程,是怎么算出-1/2的。

Answer 1

=cos(54+18)-cos(54-18)
=cos54cos18-sin54sin18-cos54cos18-sin54sin18
=-2sin54sin18
=-2cos36sin18
=-2sin18cos18cos36/(cos18)
=-sin36cos36/(cos18)
=-2sin36cos36/(2cos18)
=-sin72/(2cos18)
=-cos18/(2cos18)
=-1/2