$\begin{array}{l}
\left| {\overrightarrow {BC} } \right| + \left| {\overrightarrow {BA} } \right| = BC + BA = 5 + 12 = 17.\\
\left| {\overrightarrow {BA} } \right| - \left| {\overrightarrow {BC} } \right| = 5 - 12 = - 7.\\
Em\,\,xem\,\,lai\,\,de\,\,bai\,\,\,cua\,\,\,e\,\,co\,\,phai\,\,nhu\,\,nay\,\,k\,\,hay\,\,\,nhu\,\,nay\,\,nhe:\\
a)\,\,\left| {\overrightarrow {BA} + \overrightarrow {BC} } \right|\\
b)\,\,\,\left| {\overrightarrow {BA} - \overrightarrow {BC} } \right|.
\end{array}$