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Lesson 58: Parts of a Circle
In this lesson we'll take a break from the topics that we've been working on, and we'll learn some basic geometry. Circles come up constantly in math, and it's important to learn some definitions involving circles, and some basic properties.
Don't forget to watch the embedded YouTube video clip for this lesson at the bottom of the page.
Take
a look at the circle at right. It's important to learn some
basic definitions involving circles, since these words will come up
again and again. First, look at the line connecting points E
and F. The line is actually called a line segment.
In math, a line is something that goes on
forever in both directions, and a line segment is just a part of a
line. We would write EF with a horizontal line over it to show
that it's a segment.
Line segment EF is called a chord. This means that it is a straight line that connects two points along the edge of the circle, but it does not go through the center, in the way that the diameter does.
Line segment ADB is called the diameter of the circle. It is a straight line through the center point, starting at A, ending at B, and running through the center point D. Note that the line didn't have to go from left to right. It could have gone from top to bottom, or diagonally, and it would still be the diameter, as long as it went right through the center of the circle. In the case, the center is point D. Any line that goes through point D and touches the edges of the circle would be considered the diameter.
Line segment CD is called the radius. It connects the center of the circle with any point on the edge. Line segments AD and BD are also each considered to be a radius of the circle. Note that the radius is always half the measurement of the diameter, and the diameter is always twice the measurement of the radius.
The distance around the edge of a circle is called the circumference. Of course you can't measure it using a ruler. Later you'll learn how, and much more about circles.
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