我来回答:1..过点C做CF‖DE,交AB于F
则AM:MC =AE:EF ,∵AM=MC ,∴AE=EF ,∵AE=1/3EB,
∴EF=1/3BE,∴BF=2FE,
∵CF‖DE,∴BC:CD=BF:FE=2:1,∴BC=2CD
2.过点C作CE‖AB交 AD 的延长线于E
则AB:CE=BD:DC,∠BAD=∠E,∵∠BAD=∠CAD,
∴∠CAD=∠E,∴AC=CE,∴AB:AC=BD:DC
我来回答:1..过点C做CF‖DE,交AB于F
则AM:MC =AE:EF ,∵AM=MC ,∴AE=EF ,∵AE=1/3EB,
∴EF=1/3BE,∴BF=2FE,
∵CF‖DE,∴BC:CD=BF:FE=2:1,∴BC=2CD
2.过点C作CE‖AB交 AD 的延长线于E
则AB:CE=BD:DC,∠BAD=∠E,∵∠BAD=∠CAD,
∴∠CAD=∠E,∴AC=CE,∴AB:AC=BD:DC