Circumference and Area of Circles

1
Circumference and Area of Circles

• By Sara Gregurash

2
Circles

• Circles are part of everyday life. They are all
  around us. Some examples of circles are flowers,
  the sun, the moon, and dish plates. It is
  important to study circles because they are
  common and it is useful to know how to figure out
  their circumference and area.
• A circle is a shape that has all points the same
  distance from its center.

Center
3
Circumference

• The circumference of a circle is the distance or
  perimeter around the outside of a circle.
• To find the circumference of a circle you have to
  multiply pi (3.14159265358979323846... ) by the
  radius squared.
• The diameter is a straight line segment that
  passes through the center of the circle. The
  radius is a line segment that joins the center of
  a circle with any point on its circumference. If
  you know the radius, then the diameter is twice
  as large.

Diameter (d)
Radius (r)
4
Finding circumference of a circle

• Pi is the ratio of the circumference of a circle
  to its diameter. For any circle, if you divide
  its circumference by its diameter, you get a
  value close to pi.
• Here is how you find the circumference of a
  circle using this circle. You have to use the
  equation
• C 2 pi r (r is the radius)
• The radius of the circle below is 9 inches.
• C 2 pi r
• C 2 pi 9 in
• C 18 in pi
• C 56.5 in

r 9 in
5
Area of a circle

• If the circle is drawn on a centimeter grid paper
  like the one shown below, the area of a circle
  can be estimated by counting the unit squares and
  parts of squares that cover the circle.
• The estimated area of this circle is 12 cm2.

6
Finding the area of a circle

• To find the area of a circle you have to multiply
  pi by the radius squared. The equation looks
  like this
• A pi r2 (r is the radius)
• Here is an example using the circle from the
  previous page. To get the actual area of this
  circle you have to to use the equation.
• A pi r2
• A pi (2 cm)2
• A pi 4 cm2
• A 12.57 cm2

r 2 cm
7
Websites

• Here are some links to lessons, worksheets, and
  exercises about the circumference and area of
  circles. You can do them online to reinforce
  what you learned from my power point
  presentation.
• Circumference Lesson and Exercises
• Area Lesson and Exercises
• More Exercises
• Worksheet

8
Have fun with circles

• These two websites have fun activities to do that
  are related to circumference and area of circles.
• Circle Crossword
• Circle Word Search

9
Circumference and Area Worksheets

• Now, that you have learned all about
  circumference and area of circles practice your
  knowledge by completing my worksheet.
• Here are the answers to my worksheet.