AD/DB=AE/ECDB/AD=EC/AEDB/AD+1=EC/AE+1即：(AD+DB)/AD=(AE+EC)/AEAB/AD=AC/AE
∵AD比DB=AE比EC∴AD×EC=AE×DB∴EC比DB=AE比AD∵∠A=∠A∴△ADE相似△ABC∴AD/AB=AE/AC
