Rectangle
A rectangle is a shape with four sides and four corners. The corners are all right angles. It follows that the lengths of the pairs of sides opposite each other must be equal.
People make many rectangular things, including most tables, boxes, books, and papers.
A rectangle with all four sides equal in length is called a square.
Formulas
If a rectangle has length [math]\displaystyle{ \ell }[/math] and width w
- it has area [math]\displaystyle{ K = \ell w\, }[/math],
- it has perimeter [math]\displaystyle{ P = 2\ell + 2w = 2(\ell + w)\, }[/math],
- each diagonal has length [math]\displaystyle{ d=\sqrt{\ell^2 + w^2} }[/math],
- it has an aspect ratio = [math]\displaystyle{ \ell }[/math] : w,
- and when [math]\displaystyle{ \ell = w\, }[/math], the rectangle is a square.