合分比定理：如果a/b=c/d,则（a+b)/(a-b)=(c+d)/(c-d) (a≠b,c≠d)
e/1=（cosa/2cosb/2 - sina/2sinb/2)/(cosa/2cosb/2+sina/2sinb/2)
(e+1)/(e-1)= [cosa/2cosb/2 -sina/2sinb/2+cosa/2cosb/2+sina/2sinb/2]/[cosa/2cosb/2-sina/2sinb/2-(cosa/2cosb/2+sina/2sinb/2)]
=2cosa/2cosb/2/(-2sina/2sinb/2)
= -(cosa/2cosb/2 )/(sina/2sinb/2)