Question 394928
<pre><font face = "batangche" color = "indigo" size = 4><b>

1. If both pairs of opposite sides of a quadrilateral are parallel, the
quadrilateral is a parallelogram. (That's the definition of a parallelogram,
the other 5 are theorems that can be proved.)

2. If both pairs of opposite sides of a quadrilateral are congruent, the
quadrilateral is a parallelogram.

3. If one pair of opposite sides of a quadrilateral are both parallel and
congruent, the quadrilateral is a parallelogram.

4. If the diagonals of a quadrilateral bisect each other, the quadrilateral
is a parallelogram.

5. If both pairs of opposite angles of a quadrilateral are congruent, the
quadrilateral is a parallelogram. 

6. If every pair of adjacent angles of a quadrilateral are supplementary, the
quadrilateral is a parallelogram.

Edwin</pre>