Prove that, if the opposite sides of a quadrilateral are equal and parallel, it is a parallelogram

Exercise: Prove that, if the opposite sides of a quadrilateral are equal and parallel, it is a parallelogram. General enunciation: We have to prove that if the opposite sides of a quadrilateral are equal and parallel, it is a parallelogram. Particular enunciation: Let the opposite sides of the quadrilateral ABCD are equal and parallel .i.e. ...