As shown in the figure, M is the midpoint of line AB, point C is on line AB, and AC = 4cm, n is the midpoint of AC, Mn = 3cm, then line ab=______ cm.

As shown in the figure, M is the midpoint of line AB, point C is on line AB, and AC = 4cm, n is the midpoint of AC, Mn = 3cm, then line ab=______ cm.

As shown in the figure, ∵ m is the midpoint of line AB, n is the midpoint of AC, ∵ AB = 2am, an = 12ac, and AC = 4cm, ∵ an = 2, ∵ am = an + nm = 2 + 3 = 5 (CM), ∵ AB = 2 × 5cm = 10cm