As shown in the figure, ab = 12cm, point C is the midpoint of AB, and point D is the midpoint of line CB

As shown in the figure, ab = 12cm, point C is the midpoint of AB, and point D is the midpoint of line CB

∵ AB = 12cm, point C is the midpoint of AB, ∵ AC = CB = 12ab = 12 × 12 = 6cm, ∵ point D is the midpoint of line CB, ∵ CD = 12bc = 12 × 6 = 3cm, ∵ ad = AC + CD = 6 + 3 = 9cm