tanC/tanA+tanC/tanB=(sinCcosA)/(sinAcosC)+(sinCcosB)/(sinBcosC)=(sinCcosA-sinAcosC)/(sinAcosC)+(sinCcosB-sinBcosC)/(sinBcosC)-2=sin(A+C)/(sinAcosC)+sin(B+C)/(sinBcosC)-2=sinB/(sinAcosC)+sinA/(sinBcosC)-2=(b/a+a/b)/cosC-2=6-2=4