如何直接得出cos36-cos72=1/2 cos36*cos72=1/4?
问题描述:
如何直接得出cos36-cos72=1/2 cos36*cos72=1/4?
答
cos36-cos72解1.Cos36-cos72=(sin36cos36-sin36cos72)/sin36 =-(sin108-sin36-sin72)/2sin36 =-(sin72-sin36-sin72)/2sin36 =sin36/2sin36 =1/2解2.cos36-cos72 =cos(54-18)-cos(54+18) =cos54cos18+sin54sin18-[cos...