tana/(tana-1)=-1
∴ tana=-tana+1
∴ 2tana=1
∴ tana=1/2
∴ (sina+cosa)/(3sina-2cosa)
分子分母同时除以cosa
= (tana+1)/(3tana-2)
=(1/2+1)/[3*(1/2)-2]
=(3/2)/(-1/2)
=-3