Centroid Formula

In the sketch below, G is the centroid for the triangle.
Drag points A, B, and C around, observing their coordinates.
Observe how the coordinates of G change as A, B, and C are changed.
Determine a "formula" that can be used to determine the coordinates of the centroid, given the coordinates of the vertices of a triangle.

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Veronica Robinson, Created with GeoGebra