Pre-Algebra Demystified

SOLUTION

d¼2r

¼ 2 14 inches

¼28 inches

In order to find the circumference of a circle, a special number is used.
That number is called pi, and its symbol isp, wherep¼ 3 :141592654.... For
our purposes, we useprounded to 3: 14 :pis the number of times that a
diameter of a circle will fit around the circle.

The circumference of a circle then isC¼pd orC¼ 2 pr, where r is the

radius of the circle.

EXAMPLE

Find the circumference of a circle whose diameter is 7 inches. Usep¼ 3 :14.

SOLUTION

C¼pd

¼ 3 : 14 7 inches

¼ 21 :98 inches

EXAMPLE

Find the circumference of a circle whose radius is 12 feet. Usep¼ 3 :14.

SOLUTION

C¼ 2 pr

¼ 2  3 : 14 12 feet

¼ 75 :36 feet

Math Note: Sometimes

22

7

is used as an approximate value forp.

PRACTICE

1. Find the perimeter of a triangle whose sides are 8 feet, 9 feet, and
   11 feet.


2. Find the perimeter of a square whose side is 3 yards.


3. Find the perimeter of a rectangle whose length is 28 inches and whose
   width is 16 inches.

CHAPTER 9 Informal Geometry 169