sinacosb+cosasinb=1/2
sinacosb-cosasinb=1/3
所以sinacosb=5/12
cosasinb=1/12
相除
(sina/cosa)*(cosb/sinb)=5
(sina/cosa)/(sinb/cosb)=5
tana/tanb=5
sin(a+b)=sina*cosb+sinb*cosa=1/2
sin(a-b)=sina*cosb-cosasinb=1/3
sina*cosb=5/12 sinb*cosa=1/12
tana/tanb=sina/cosa*sinb/cosb=sina*cosb/cosa*sinb=5