Як довести, що чотирикутник - паралелограм?

There are four basic definitive properties of a true parallelogram

1. Both pairs of opposite sides are parallel

2. Both pairs of opposite sides are of the same length

3. At least one pair of opposite sides is both parallel and the same length

4. Both pairs of opposite angles are the same

However, if any one of these properties is known to be true, then a quadrilateral is a parallelogram.