思路：分别延长AD、A`D`至E与E`使DE=AD,D`E`=A`D`,
易证：△ABD≌△ECD △A`B`D`≌△E`C`D`
得EC=AB AE=2AD ∠BAD=∠E
E`C`=A`B` A`E`=2A`D` ∠B`A`D`=∠E`
由AB:A'B'=AC:A'C'=AD:A'D'得EC:E'C'=AC:A'C'=AE:A'E'
∴△AEC∽△A'E'C'
∴∠E=∠E` ∠EAC=∠E'A'C'
即∠BAD=∠B`A`D`
∴∠BAC=∠B`A`C`由AB:A'B'=AC:A'C'
∴△ABC∽△A'B'C