Elementary and Intermediate Algebra: Concepts & Applications (6th Edition)

perimeter = $2l+2w$
A rectangle is a 4-sided figure where opposite sides are equal in length. The perimeter of a rectangle is the sum of the lengths of the sides. If the length of a rectangle is l, there are 2 sides that have a length of l. If the width of a rectangle is w, there are 2 sides that have a width of w. The sum of the lengths of the 4 sides is then $l+l+w+w$ or $2l+2w$.