tanA/tanB=sinAcosB/(cosAsinB)根据正弦定理：(2c-b)/b=2c/b-1=2sinC/sinB-1,所以sinAcosB/(cosAsinB) =2sinC/sinB-1,去掉分母：sinAcosB=2sinCcosA-cosAsinB 即sin（A+B）=2sinCcosAsinC = 2sinCcosA cosA=1/2 A=60...