Hello,
Here i am using vectors
[tex]\vec{AB}=\vec{DC}=3\vec{i}+3\vec{j}[/tex]
==> AB//DC and AD//BC (definition of a translation)
The quadrilater ABCD is a parallelogram:
so
1) his diagonals have the same length;
2) his diagonals are cutting in the middle.
(these are proprieties of a parallelogram)
