So an = NC = 1AC.

Because m is the midpoint of CB.

So cm = MB = 1CB.

Because MN=NC+MC

And because NC = 1AC.

MC = 1CB。

So Mn = 1ac+ 1CB.

So Mn = 1(AC+CB)

Because AB=AC+CB

Because AB=8

So Mn = 1 times 8.

=4