Đáp án:
$\begin{array}{l}
6)\left| {\overrightarrow a + \overrightarrow b } \right| = \sqrt {{5^2} + {{12}^2} + 2.5.12.c{\rm{os}}{{90}^0}} = \sqrt {{5^2} + {{12}^2}} = 13\\
\left| {\overrightarrow a - \overrightarrow b } \right| = \sqrt {{5^2} + {{12}^2} - 2.5.12.c{\rm{os}}{{90}^0}} = \sqrt {{5^2} + {{12}^2}} = 13
\end{array}$