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