Question: what additional information is needed in order to prove that quadrilateral abcd is a parallelogram?
Answer: If one pair of opposite sides of a quadrilateral are congruent and parallel, then the quadrilateral is a parallelogram. If — BC — AD and — BC ≅ — AD , then ABCD is a parallelogram. If the diagonals of a quadrilateral bisect each other, then the quadrilateral is a parallelogram.