正弦定理,a=RsinA,b=RsinB,R为外接圆半径代入a^2/b^2=sin^2A/sin^2B而tanA/tanB=a^2/b^2即sinAcosB/sinBcosB=a^2/b^2即sinAcosB/sinBcosB=sin^2A/sin^2B即sinA/sinB=cosB/cosA即sinAcosA=sinBcosB即A=B即三角形为等...