perimeter

A perimeter is the length of a closed boundary that encompasses, surrounds, or outlines either a two dimensional shape or a one-dimensional line. The perimeter of a circle or ellipse is called its circumference.

Examples

name	perimeter formula	variable definition
circle	$$2\pi r = \pi d$$	$r$ : radius $d$ : diameter
semicircle	$$(\pi+2)r$$	$r$ : radius
triangle	$$a+b+c$$	$a, b, c$ : sides of the triangle
rectangle	$$2(l+w)$$	$l$ : length $w$ : width
square/rhombus	$$4l$$	$l$ : side length
regular polygon	$$n\times a$$	$n$ : number of sides $l$ : side length