AM/BM=1/2,AN/BN=3/2
所以AM:AB=1/2÷(1/2+1)=1/3
AN:AB=3/2÷(3/2+1)=3/5
所以AN-AM=(3/5-1/3)AB=4/15AB
而AN-AM=MN=4
所以AB=4÷4/15=15