很简单,首先画个图,∵AB⊥AC∴∠BAC=90°∵AC⊥DC∴∠ACD=90°∴∠BAC=∠ACD=90°∵AD=BC,AC=CA∴△ACD全等于△CAB（HL）∴DC=AB又∵∠BAC=∠ACD=90°∴AB∥DC（AB=DC）∴四边形ABCD是平行四边形∴AD∥BC图图我不好...