The coordinates of the vertices of a triangle are: $\left(x_{1}, y_{1}\right),\left(x_{2}, y_{2}\right),\left(x_{3}, y_{3}\right)$. What are the coordinates of the centroid of the triangle?
$$
\begin{aligned}
& x=\frac{1}{3}\left(x_{1}+x_{2}+x_{3}\right) \\
& y=\frac{1}{3}\left(y_{1}+y_{2}+y_{3}\right) .
\end{aligned}
$$

(See: K. M. L. Vol. VI, No. 616 problem.)