Some theorems using vectors.

## Circumcentre and orthocentreEdit

For a triangle ABC, let the circumcentre be O and the orthocentre be H. Then the vector sum of **OA**, **OB** and **OC** is **OH**.

## CentroidEdit

For a triangle ABC, let the centroid be G. Then the vector sum of **GA**, **GB** and **GC** is **0**.

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