Let O and r be the center and the radius of the incircle, as shown in the figure.
Given AB+BC+CA=S
Let |ABC| denote area of DeltaABC, => |ABC|=1/2*r*(AB+BC+CA)=1/2rS
given DE divides DeltaABC into two equal areas, => |CDE|=1/2|ABC|=1/4rS ----- Eq(1) |CDE|=1/2*r*(CD+CE) ----- Eq(2)
As Eq(2)=Eq(1), => 1/2*r*(CD+CE)=1/4rS => CD+CE=S/2