You can put this solution on YOUR website! It is always true.
A parallelogram is a quadrilateral where opposite sides are parallel.
So a quadrilateral ABCD is a parallelogram if and only if AB is parallel to DC,
and BC is parallel to AD.
The angles between side AB and adjacent sides AD and BC are supplementary (their measures add up to .
The angles between side CD and adjacent sides AD and BC are supplementary.
The angles at opposite vertices are congruent.
There are only two interior angle measures for a parallelogram, and those two angle measures add up .
In the figure above,
the angles at A and C are a pair congruent angles (they have the same measure);
the angles at B and D are another pair of congruent angles,
and any angle of one of those pairs is supplementary to any angle from the other pair.
If all the angles in a parallelogram have the same measure, they all measure , and the parallelogram is a rectangle, which is a special kind of parallelogram.
