连接AC,AD,
AB=AE,
∠B=∠E,
BC=ED ,
△ABC≌△AED,[SAS]
∠ACB=∠ADE,
AC=AD,
∠ACD=∠ADC,
∠ACB+∠ACD=∠ADE+∠ADC,
∠BCD=∠EDC,(∠C=∠D)