In a circle:

The Radius is the distance from the center outwards.

The Diameter goes straight across the circle, through the center. Radius is always ½ or half of the diameter.

The Circumference is the distance of going once around the circle. The relationship of circumference and radius is expressed in the following formula:

If you know the radius of a circle, you will also be able to know its area, as:

Radius and circumference are not the only lines that ?touch? the circle. There are more types of lines that ?touch? circles:

·         A line that "just touches" the circle as it passes by is called a Tangent.

·         A line that cuts the circle at two points is called a Secant.

·         A line segment that goes from one point to another on the circle's circumference is called a Chord.

·         The above are straight lines. Here is the name for a special non-straight line: arc, which is any part of the circumference.

A circle can also be cut into "slices". And generally there are 2 ways of "slicing" a circle:

·         You can slice a circle with radius, and this slice is called "Sector".

·         You can also slice a circle with chords, and this slice is called "Segment".

And remember: just as a square is a special case of rectangle, a circle is just a special case of ellipse: