1. In triangle ABC, D is the midpoint of AB and E is the midpoint of AC.  BE and CD intersect at point F.

Given that  $\overset{\rightarrow}{AB}=2a$ and  $\overset{\rightarrow}{AC}=2b$, write the vectors $\overset{\rightarrow}{BC},\overset{\rightarrow}{BE}$and $\overset{\rightarrow}{CD}$ in terms of a and b.