Step-by-step explanation:
If segment AD contains point B and, then A, B, C and D lies on the same straight line as shown in the attachment.
To show that point C is the middle of point BD, we musr show that segment BC is equal to CD i.e BC = CD.
From the diagram, AC = AB+BC
Given AC = 12cm and AB = 4cm
BC = AC-AB
BC = 12cm - 4cm
BC = 8cm
Also AD = AB+BC+CD
Given AD = 20cm, AB = 4cm and BC = 8cm
CD = AD-(AB+BC)
CD = 20-(4+8)
CD = 20-12
CD = 8cm
It can be seen that BC = CD = 8cm, hence C is the midpoint of B and C.