Quadrilateral
A polygon with exactly four sides and four vertices.
Parallelogram
A quadrilateral containing two pairs of parallel sides.
Diagonal of a polygon
A line segment that connects the nonconsecutive vertices of a polygon.
Consecutive angles are angles of a polygon that share a side.
Parallelograms have special properties regarding side lengths, angles, and diagonals. We can use these properties to find unknown angles or sides of parallelograms. The following are theorems about parallelograms:
Parallelogram opposite sides theorem
If a quadrilateral is a parallelogram, then its opposite sides are congruent.
Parallelogram opposite angles theorem
If a quadrilateral is a parallelogram, then its opposite angles are congruent.
Parallelogram consecutive angles theorem
If a quadrilateral is a parallelogram, then its consecutive angles are supplementary.
Example: \angle ADC and \angle DAB are supplementary
Parallelogram diagonals theorem
If a quadrilateral is a parallelogram, then its diagonals bisect each other.
To prove that a quadrilateral is a parallelogram, we can make use of the following theorems:
Parallelogram opposite sides converse
If both pairs of opposite sides are congruent, then the quadrilateral is a parallelogram.
Parallelogram opposite angles converse
If both pairs of opposite angles are congruent, then the quadrilateral is a parallelogram.
Parallelogram consecutive angles converse
If an angle is supplementary to both of its consecutive angles, then the quadrilateral is a parallelogram.
Parallelogram diagonals converse
If a quadrilateral has diagonals that bisect each other, then the quadrilateral is a parallelogram.
Opposite sides parallel and congruent theorem
If one pair of opposite sides of a quadrilateral are congruent and parallel, then the quadrilateral is a parallelogram.