Circles: Circumference & AreaTutorial 8c

A farmer might need to find the area of his circular field to calculate irrigation costs.

Circumference Circumference is the distance around a circle.

Similar to the perimeter of a polygon. The circumference of a circle is slightly more

than three times the length of its diameter (actually 3.1415 . . .)

Circumference The number 3.1415 . . . Goes on forever

without repeating. We have come to refer to this number as

pi (). It is represented by the Greek symbol .

As a fraction, is approximately 22/7. As a decimal, is approximately 3.14.

Circumference To find the circumference of a circle we use one

of the following formulas. Circumference(c) = times the diameter(d)

C = d Or since the diameter is twice the radius-

C = 2r

Diameter

Example #1 Find the approximate circumference if the

diameter has a length of 5 inches.

Diameter

5”

C = d = •5 (3.14) •5 15.7

The circumference is approximately() 15.7 inches

Area of a circle The formula to find the area of a circle is pi

() times the radius (r) squared. A = r2

Diameter

14”

Example:Since the diameter is 14, the radius must be 7.

A = r2

= •72

= •7 •7 = (3.14) • 49 = 153.86 square feet