由:tanA/tanB=(2c-b)/b
得:1+tanA/tanB=2c/b
1+[sinAcosB]/[sinB/cosA]=2sinC/sinB
sinBcosA+sinAcosB=2sinCcosA
sin(A+B)=2sinCcosA
sinC=2sinCcosA
cosA=1/2
∴A=60°