在Rt△ABC与Rt△A1B1C1中,∠A=∠A1=90°,AD⊥BC,A1D1⊥B1C1,垂足分别为点D、D1,且AB/A1B1=AD/A1D1.

在Rt△ABC中,∠A=90°,AD⊥BC
所以Rt△ABC∽Rt△ABD∽Rt△ACD
AB/BC=AD/AC=BD/AB
转化为AB/AD＝BC/AC
同理可得Rt△A1B1C1∽Rt△A1B1D1∽Rt△A1C1D1
A1B1/B1C1=A1D1/A1C1=B1D1/A1B1
转化为A1B1/A1D1＝B1C1/A1C1
因为AB/A1B1=AD/A1D1 即AB/AD=A1B1/A1D1
所以AB/A1B1=BC/B1C1=AC/A1C1
所以：△ABC∽△A1B1C1