(a) In line segments AE and EC, point E is a common point
So, AE + EC = AC
(b) In part (a) we have proved that:
AE + EC = AC
⇒ AC – EC = AE
(c) For line segments BE and ED, point E is a common point.
So, BE + ED = BD
⇒ BE – BE = ED
(d) Also, BE = BE + ED
⇒ BD – DE = BE
(∵ line segment ED = line segment DE)