如图：已知线段AB=15cm,c点在AB上,BC=三分之二AC,D为BC的中点,求BC、AD的长

AB = 10
C 是AB的中点
BC = AC = 10/2 = 5
D 是BC的中点
CD = DB = 5/2 =2.5
AD = AC + CD
AD = 5 + 2.5 = 7.5
答：AD的长度是7.5cm.