(tanA-tanB)/(tanA+tanB)=(sinAcosB-cosAsinB)/(sinAcosB+cosAsinB)=(sinAcosB-cosAsinB)/sin(A+B)=(sinAcosB-cosAsinB)/sinC…①.正弦定理(b+c)/c=(sinB+sinC)/sinC…②,由①,②得sinAcosB-cosAsinB=sinB+sinC=sinB...注意(b+c)/a而不是(b+c)/c答案粘贴要有度( ⊙ o ⊙ )啊！