证明:在BC上截取CE=CA,并连接DE
∵∠1=∠2
CD=CD
∴△CAD全等于△CED
∴∠A=∠CED
AD=ED
∵∠CED=∠B+∠BDE
∠A=2∠B
∴∠CED=2∠B=∠B+∠BDE
∴∠B=∠BDE
∴BE=DE=AD
∵BC=BE+CE
∴BC=AC+AD