In the triangle ABC depicted at right, E is the midpoint of the side BC and D is some point on the segment AE. Let DA = a, DB = b, and DC = a + b. Express the following vectors in terms of a, b, and c:
(a) AB = B - A
(b) DE = DF + DC + CE
If |DE| = √(a^2 + b^2), find a + b + c/2.