The diagonals of a parallelogram bisect each other. Steps
(a), (b), and (c) outline a proof of this theorem. (See Exercise 25 for a particular instance of this theorem.)
(a) In the parallelogram OABC shown in the figure, check that the coordinates of $B$ must be $(a+b, c)$
(b) Use the midpoint formula to calculate the midpoints of diagonals $\overline{O B}$ and $\overline{A C}$
(c) The two answers in part (b) are identical. This shows that the two diagonals do indeed bisect each other, as we wished to prove.
(FIGURE CAN'T COPY)