Ta có: $\left \{ {{x_{G} =\frac{x_{A} +x_{B} +x_{C}}{3} } \atop {Y_{G} =\frac{Y_{A} +Y_{B} +Y_{C}}{3}}} \right.$
$⇔$ $\left \{ {{x_{G} =\frac{1 +2 -1}{3} } \atop {Y_{G} =\frac{-2 +3+-2}{3}}} \right.$
$⇔$ $\left \{ {{x_{G} =\frac{2}{3} } \atop {Y_{G} =\frac{-1}{3}}} \right.$
Vậy $x_{G} +y_{G} = \frac{2}{3} + \frac{-1}{3} = \frac{1}{3}$