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Notes on Circles


Circumference: The distance around the circle

The formula for circumference is or

          Pi times the diameter of the circle

where D is the diameter and r is the Radius

            D= 42' so Circumference is 42 3.14=131.88 ft

That is the approximate circumference since Pi is not exactly equal to 3.14.  To find the Exact circumference we leave our answer in terms of Pi.  For example when the diameter is 42 the exact circumference is 42P

Find the circumference of the following circle using P = 3.14




Find the exact circumference of the following circles (leave in terms of Pi)

 2)                            3)

Area of a circle: The space inside the circle.

The formula for area is

where r is the Radius: You multiply Pi times the radius squared

In the 1st example the diameter was 42 feet so the radius is 21 ft. 

area = 212 x 3.14

area = 441 x 3.14= 1384.74 ft2

Note area is always expressed in square units

For practice use the formula above to calculate both the approximate area using 3.14 for Pi and the actual area (in terms of Pi)