If B is the mid point of (AC) and C is the point of (BD) where A, B, C, D lie on a straight line, say why AB = CD?

∵ B is the mid-point of AC
∴ AB = BC …(1)
∵C is the mid-point of BD
∴ BC = CD …(2)
In view of (1) and (2), we get
AB = CD.