Trigonometry/Some theorems using vectors

Some theorems using vectors.

Circumcentre and orthocentre edit

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.

Centroid edit

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