Rectangle

A closed planar Quadrilateral with opposite sides of equal lengths $a$ and $b$, and with four Right Angles. The Area of the rectangle is

$$A=ab,$$

and its Diagonals are of length

$$p,q=\sqrt{a^2+b^2}\,.$$

A Square is a degenerate rectangle with $a=b$.

See also Golden Rectangle, Perfect Rectangle, Square

References

Beyer, W. H. (Ed.) CRC Standard Mathematical Tables, 28th ed. Boca Raton, FL: CRC Press, p. 122, 1987.

Eppstein, D. ``Rectilinear Geometry.'' http://www.ics.uci.edu/~eppstein/junkyard/rect.html.