14. In trapezoid $A B C D$, $A B / / C D, A B=2 C D, M$ and $N$ are the midpoints of $D C$ and $A B$ respectively. Given $\overrightarrow{A B}=\vec{a}, \overrightarrow{A D}=\vec{b}$, find $\overrightarrow{B C}$ and $\overrightarrow{M N}$.
14. $\overrightarrow{B C}=\vec{b}-\frac{1}{2} \vec{a} ; \overrightarrow{M N}=\frac{1}{4} \vec{a}-\vec{b}$.