由a/sina=b/sinb=c/sinc=2R
sina+sinc=2sinb
2sin[(a+c)/2]*cos[(a-c)/2]=2sin(a+c)
2sin[(a+c)/2]*cos[(a-c)/2]=2*2sin[(a+c)/2]*cos[(a+c)/2]
cos[(a-c)/2]=2cos[(a+c)/2]
cosa/2*cosc/2+sina/2*sinc/2=2cosa/2*cosc/2-2sina/2*sinc/2
cosa/2*cosc/2=3sina/2*sinc/2
ctga/2*ctgc/2=3