The Circle
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A circle is a round shape. All the points on the edge, (the edge is called the circumference), are the same distance from the center. We use a large C to represent the circumference.

The diameter of a circle (we use a small d to represent the diameter) is the distance across the circle, passing through its center. It is a line that connects two points on the circle's circumference. The line must  pass through the center.

The radius of a circle, (we use a small r to represent the radius), is the distance from the center of the circle to any point on its circumference. It is half the length of the diameter, which is the distance across the circle through the center.



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