Here's the question you clicked on:
steanson
The diagonals of a plane quadrilateral ABCD intersect at O, and X,Y are the mid points of the diagonals AC and BD respectively. Show that: (vectors) (i) 2AB + 2BC + 2CA = O and (ii) If OA + OB + OC + OD = 4OM, find the location of M
Now (i) doesn't look right to me as O is not a vector? Any one shed any light on this
1) I think it is not O but 0 (zero) ABC form a triangle hence AC = AB + BC (vector addition). Hence AB +BC+CA = 0
yes i think you're right, its def a typo in the book
the last part turns out to be mid XY not sure how to prove that? I'll post the question again...