$\begin{array}{l}
a)\,\,\,\overrightarrow {HA} - 5\overrightarrow {HB} + \overrightarrow {HC} \\
= \overrightarrow {HG} + \overrightarrow {GA} - 5\left( {\overrightarrow {HG} + \overrightarrow {GB} } \right) + \overrightarrow {HG} + \overrightarrow {GC} \\
= \overrightarrow {HG} + \overrightarrow {GA} - 5\overrightarrow {HG} + \overrightarrow {HG} - 5\overrightarrow {GB} + \overrightarrow {GC} \\
= - 3\overrightarrow {HG} + \left( {\overrightarrow {GA} + \overrightarrow {GB} + \overrightarrow {GC} } \right) - 6\overrightarrow {GB} \\
= - 3\overrightarrow {HG} - 6\overrightarrow {GC} \\
= - 3\overrightarrow {HG} + 3\overrightarrow {HG} = \overrightarrow 0 \,\,\,\left( {dpcm} \right)\\
Cau\,\,b\,\,\,la\,\,theoAG\,\,va\,\,\,AH\,\,a\,\,e?
\end{array}$
