Circumference and area of circles
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Circumference and Area of Circles - PowerPoint PPT Presentation

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  • 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.



  • 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)

Finding circumference of a circle
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

Area of a circle
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.

Finding the area of a circle
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


  • 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


Have fun with circles
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

Circumference and area worksheets
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.