Basic Math & Pre-Algebra For Dummies
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The center of a circle is a point that's the same distance from any point on the circle itself. This distance is called the radius of the circle, or r for short. And, any line segment from one point on the circle through the center to another point on the circle is called a diameter, or d for short.


The diameter

As you can see, the diameter of any circle is made up of one radius plus another radius — that is, two radii (pronounced ray-dee-eye). This concept gives you the following handy formula:


For example, given a circle with a radius of 5 millimeters, you can figure out the diameter as follows:


The circumference

Because the circle is an extra-special shape, its perimeter (the length of its "sides") has an extra-special name: the circumference (C for short). Early mathematicians went to a lot of trouble to figure out how to measure the circumference of a circle. Here's the formula they hit upon:


Note: Because 2 x r is the same as the diameter, you also can write the formula as C = π x d.

The symbol π is called pi (pronounced "pie"). It's just a number whose approximate value is as follows (the decimal part of pi goes on forever, so you can't get an exact value for pi):


So given a circle with a radius of 5 mm, you can figure out the approximate circumference:


The area of a circle

The formula for the area (A) of a circle also uses π:


Here's how to use this formula to find the approximate area of a circle with a radius of 5 mm:


About This Article

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Mark Zegarelli earned degrees in mathematics and English from Rutgers University. He is the founder of SimpleStep Learning, an online educational platform that teaches courses in basic concepts in ten minutes or less, keeping students engaged and learning in every moment. Mark is also author of several other successful For Dummies books.

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