sin(a+b)/sin(a-b)=(sina*cosb+cosa*sinb)/(sina*cosb-cosa*sinb)
分式上下同除以cosa*cosb,则
sin(a+b)/sin(a-b)=(tana+tanb)/(tana-tanb)
=(tana/tanb+1)/(tana/tanb-1)=(2/3)/(-1/5)
所以tana/tanb=7/13