∵AC=CD=DB,BM=2BN,AM=MC
∴BN=MN=BD+DN=(1/3)AB+DN=5;
∵M(jìn)N=CD-DN+CM=(1/3)AB-DN+CM=5;
∵AM=AC-MC=(1/3)AB-MC
∴MC=(1/6)AB;
∴BM=BN+NM=(2/3)AB+CM=10;
∴AB=12;
∴ND=BN-(1/3)AB=5-4=1;
∴CN=CD-ND=(1/3)AB-ND=4-1=3;