# Area of a Circle

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The distance around a circle is called its circumference. The distance across a circle through its center is called its diameter. We use the Greek letter Pi (pronounced Pi) to represent the ratio of the circumference of a circle to the diameter.

In the last lesson, we learned that the formula for circumference of a circle is: C = Pi x d. For simplicity, we use Pi = 3.14. We know from the last lesson that the diameter of a circle is twice as long as the radius. This relationship is expressed in the following formula: d = 2 x r.

The area of a circle is the number of square units inside that circle. If each square in the circle to the left has an area of 1 cm2, you could count the total number of squares to get the area of this circle. Thus, if there were a total of 28.26 squares, the area of this circle would be 28.26 cm2 However, it is easier to use one of the following formulas:

A = Pi x r2 or A = Pi x r x r

Where A is the area, and r is the radius. Let's look at some examples involving the area of a circle. In each of the three examples below, we will use Pi = 3.14 in our calculations.