Basic Math and Pre-Algebra

PART 3. The Shape of the World


CHAPTER 14. Quadrilaterals and Other Polygons

Perimeter of Quadrilaterals


It’s easy to calculate the perimeter of quadrilaterals. Just as with triangles, you simply add up the lengths of the sides. Most figures don’t even have a special formula for perimeter.

For a rectangle, you see the formula P = 2L + 2W. Because the opposite sides are the same length, you have two of the lengths and two of the widths. For a square or a rhombus, that becomes P = 4s. All the sides are the same length, and you have four of them.

Let’s look at a story problem using perimeter.

Suppose Marianna wants to build a fence around her vegetable garden. If the garden is a rectangle 30 feet long and 15 feet wide, and fencing costs $1.25 per foot, how much will it cost to fence the garden?

The perimeter of a rectangle is P = 2L + 2W, so she will need (2 x 30) + (2 x 15) = 60 + 30, or 90 feet of fencing. 90 feet of fencing at $1.25 per foot will cost 90 x 1.25 = $112.50. Marianna’s fence will cost $112.50.