1. How to calculate the area of a circle.
It’s as easy as pi.

2. Let’s first make sure that we understand the difference between circumference and area.

3. The circumference of a circle is the perimeter of the circle.

4. Imagine that the circle is straightening itself out.

8. The length of this line segment is the circumference of the circle.
314 cm

9. The circumference is the same length as 3 diameters plus
The circumference is the same length as 3 diameters plus .14 of another diameter.

10. So, circumference = diameter x 3.14

15. Does this look familiar?

16. O.K., now it’s time to move forward with some new stuff.

17. How in the world would you find the area of a circle?

18. Remember, area is always measured in square units.

19. Remember that the area of a rectangle is length x width because you’re calculating the total number of squares inside of the rectangle. 2 4

20. That’s fine and dandy, but a circle is not a polygon
That’s fine and dandy, but a circle is not a polygon. It does not have straight sides; it has curves.

21. How are we going to get around these curves?

22. Imagine chopping up the circle as if it were a pizza.

23. Now, let’s rearrange our “pizza” into another shape.

31. PRESTO!

32. Great Mr. Dunlap! But what in the world is this?

33. Believe it or not, this is really our “friend” the parallelogram
Believe it or not, this is really our “friend” the parallelogram. And, we know how to calculate the area of a parallelogram.

34. Rats! He always has an answer for everything.

35. Area = Base x Height
Height
Base

36. To find the area of the circle (which is now a parallelogram), we just need to multiply the Base by the Height.
Height
Base

37. Wait a minute! The height of this “parallelogram” is really the radius of the circle.
Base

38. Wait a minute! The Base is really 1/2 of the circumference.
Radius
1/2 of Circumference

39. Wait a minute! The circumference is really Diameter x 
Radius
1/2 of Diameter x 

40. Wait a minute! 1/2 of a Diameter is really a Radius.
Radius x 

41. So if we multiply the Base x Height

42. We are really multiplying Radius x Radius x 

43. Practice Time!

44. 1) Now let’s try this formula. Find the area of this circle.
5 cm

45. 5 x 5 x 3.14 = 78.5 square cm
5 cm

46. 2) Find the area of this circle.
6 cm

47. 6 x 6 x 3.14 = square cm
6 cm

48. 3) Find the area of this circle.
9 cm

49. 9 x 9 x 3.14 = square cm
9 cm

50. 4) Find the area of this circle.
20 cm

51. Make sure that you use the radius of the circle.
10 x 10 x 3.14 = 314 cm2
Make sure that you use the radius of the circle.
20 cm

52. 5) Find the area of this circle.
14 cm

53. Make sure that you use the radius of the circle.
7 x 7 x 3.14 = cm2
Make sure that you use the radius of the circle.
14 cm

54. 6) Find the area of this circle.
22 cm

55. 11 x 11 x 3.14 = cm2
22 cm

56. Area = Radius x Radius x 
It’s as easy as pi.