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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 cm ^{2}**, 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 cm**However, it is easier to use one of the following formulas:

^{2}**A = Pi x r**or

^{2}**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.