第三题慢慢做
DC=2 CE=3 DE=5 DF=2.5 CF=0.5
DE=DC+CE=1/2(AC+CB)=1/2AB
CF=CE-EF=1/2CB-1/2DE=1/2CB-1/4AB=1/16AB
1/2CB=5/16AB AC/CB=(AB-CB)/CB=AB/CB-1=3/5
3.CP+1/8AB=1/4AC CP=1/4AC-1/8AB=1/4AC-1/8AC-1/8CB=1/8(AC-CB)=1/8a
