按线段起点和终点来思考：（以下均为线段长度）AB = AC + CD + DB -->AC + BD = AB - CD = 15 - 6 = 9cm;AB = AM + MN + NB -->MN = AB - (AM+NB) = AB - (AC+BD)/2 /*MN为中点*/ = 15 - 9/2 = 21/2 = 10.5cm#
