Possible Duplicate:
A parallelogram and a line joining a vertex to the midpoint of opposite side
Proving two lines trisects a line
$\rm ABCD$ is a parallelogram (in the plane) and $\rm E$ is the midpoint of $\rm AB.$ Prove that $ \rm AC$ and $\rm DE$ trisect one another. In other words, that their point of intersection $\rm X$ divides each of them in the ratio $2 : 1.$