\[\begin{array}{l}
Goi\,M\,la\,trung\,diem\,cua\,BC.\\
Ta\,co:\left| {\overrightarrow {AB} + \overrightarrow {AC} } \right| = 2\left| {\overrightarrow {AM} } \right| = 2AM\\
Ma\,AM = AB\sin {60^0} = a.\frac{{\sqrt 3 }}{2} = \frac{{a\sqrt 3 }}{2}\\
\Rightarrow \left| {\overrightarrow {AB} + \overrightarrow {AC} } \right| = 2.\frac{{a\sqrt 3 }}{2} = a\sqrt 3 \\
\left| {\overrightarrow {AB} - \overrightarrow {AC} } \right| = \left| {\overrightarrow {CB} } \right| = CB = a
\end{array}\]
