∵∠1=∠B∴DE=BE且∠AED=180-∠DEB=180-(180-∠1-∠B)=∠1+∠B=2∠B∵AD是角平分线∴∠CAD=∠BAD又∵∠C=2∠B=∠AED,AD是公共边∴三角形ACD≌三角形AED∴AC=AE,CD=DE∴AB=AE+BE=AE+DE=AC+CD
