If M is the midpoint of the side `vec(BC)` of a triangle ABC, prove that `vec(AB)+vec(AC)` = `2vec(AM)`

Explanation

Let M is the midpoint of BC of `triangleABC`
Clearly `vec(BM)` and `vec(CM)` are equal and opposite.
Now `vec(AB)+vec(BM)` = `vec(AM)` and `vec(AC)+vec(CM)` = `vec(AM)`
`implies vec(AB)+vec(AC)+vec(BM)+vec(CM)` = `2vec(AM)`
`implies vec(AB)+vec(AC)` = `2vec(AM)`