m=(rsina-rsinb)/(rcosa-rcosb)
=(sina-sinb)/(cosa-cosb)
=[2cos(a/2+b/2)sin(a/2-b/2)]/[-2sin(a/2+b/2)sin(a/2-b/2)]
=-cot(a/2+b/2)
m=(rsina-rsinb)/(rcosa-rcosb)
=(sina-sinb)/(cosa-cosb)
=[2cos(a/2+b/2)sin(a/2-b/2)]/[-2sin(a/2+b/2)sin(a/2-b/2)]
=-cot(a/2+b/2)