$$\eqalign{ & 2b)\,\,goi\,\,G\,\,la\,\,trong\,\,tam\,\,\Delta ABC \cr & \Rightarrow \overrightarrow {GA} + \overrightarrow {GB} + \overrightarrow {GC} = \overrightarrow 0 \cr & \Rightarrow 2\overrightarrow {GA} + 2\overrightarrow {GB} + 2\overrightarrow {GC} = \overrightarrow 0 \cr & \Rightarrow \left( {\overrightarrow {GA} + \overrightarrow {GB} } \right) + \left( {\overrightarrow {GB} + \overrightarrow {GC} } \right) + \left( {\overrightarrow {GC} + \overrightarrow {GA} } \right) = \overrightarrow 0 \cr & \Leftrightarrow 2\overrightarrow {GM} + 2\overrightarrow {GP} + 2\overrightarrow {GN} = \overrightarrow 0 \cr & \Leftrightarrow 2\left( {\overrightarrow {GM} + \overrightarrow {GP} + \overrightarrow {GN} } \right) = \overrightarrow 0 \cr & \Leftrightarrow \overrightarrow {GM} + \overrightarrow {GP} + \overrightarrow {GN} = \overrightarrow 0 \cr & \Rightarrow G\,\,cung\,\,la\,\,trong\,\,tam\,\,\Delta MNP. \cr & 3)\,\,Goi\,\,G\,\,la\,\,trong\,\,tam\,\,\Delta MPE \cr & \Rightarrow \overrightarrow {GM} + \overrightarrow {GP} + \overrightarrow {GE} = \overrightarrow 0 \cr & \Rightarrow 2\overrightarrow {GM} + 2\overrightarrow {GP} + 2\overrightarrow {GE} = \overrightarrow 0 \cr & \Rightarrow \overrightarrow {GA} + \overrightarrow {GB} + \overrightarrow {GC} + \overrightarrow {GD} + 2\overrightarrow {GE} = \overrightarrow 0 \cr & \Rightarrow \left( {\overrightarrow {GB} + \overrightarrow {GC} } \right) + \left( {\overrightarrow {GD} + \overrightarrow {GE} } \right) + \left( {\overrightarrow {GA} + \overrightarrow {GE} } \right) = \overrightarrow 0 \cr & \Leftrightarrow 2\overrightarrow {GN} + 2\overrightarrow {GQ} + 2\overrightarrow {GR} = \overrightarrow 0 \cr & \Leftrightarrow \overrightarrow {GN} + \overrightarrow {GQ} + \overrightarrow {GR} = \overrightarrow 0 \cr & \Rightarrow G\,cung\,\,la\,trong\,\,tam\,\,\Delta NQR. \cr} $$