The two line segments formed by the bisector of the angle of a triangle are proportional to the two adjacent sides of the angle.

C is the extension of CN∨AB and AM in n?

And then △ABM∽△NCM?

∴AB/NC=BM/CM？

Can you prove ∠CAN=∠ANC?

∴AC=CN？

∴AB/AC=MB/MC？