MN=1.5,对吗?
∵AM:MB=5:11,∴AM=5/16AB,
∵AN:NB=5:7,∴AN=5/12AB,
∴MN=AN-AM=(5/12-5/16)AB=5/48AB,
∴5/48AB=3/2,
AB=72/5=14.4.