（a2-b2-c2）tanA+（a2-b2+c2）tanB
=-2bc*cosA*tanA+2ac*cosB*tanB
=2c(a*sinB-b*sinA)
由正弦定理,a/b=sinA/sinB a*sinB=b*sinA
=2c(a*sinB-b*sinA)=0