What is your definition of vector addition?
Complete the following non diagram proof that vector addition is associative:
For all points P, Q, R, S, (PQ + QR) + RS = PR + ...........
...........
...........
= PQ + (QR + RS)
Note that all the letters ie 'PQ' etc have an arrow above them --->, not a single one for all, but one above each letter pair.