Geometry

Quadrilaterals are four-sided plane figures. Various special quadrilaterals are defined in various ways. The following are typical:

Trapezoid:A quadrilateral with one pair of parallel sides.

Parallelogram: A quadrilateral with two pairs of parallel sides.

Rhombus: A parallelogram with four equal sides.

Kite: A quadrilateral with two pairs of equal adjacent sides.

Rectangle: A parallelogram with four right angles.

Square: A rectangle with four equal sides. It is a special kind of rhombus.

Cyclic quadrilateral: A quadrilateral whose four vertices lie on one circle.

In any quadrilateral the sum of the angles is 360^o. In a cyclic quadrilateral opposite angles are supplementary.

The diagonals of any parallelograms bisect each other. The diagonals of kites and any rhombus are perpendicular to each other.

Opposite sides of parallelograms are equal.

Figure 4. Illustration by Hans & Cassidy. Courtesy of Gale Group.