Related Topics: More Geometry Lessons

In these lessons, we will learn the following parts of a circle: diameter, chord, radius, arc and tangent

We will also learn about congruent circles, concentric circles and intersecting circles.

In these lessons, we will learn the following parts of a circle: diameter, chord, radius, arc and tangent

We will also learn about congruent circles, concentric circles and intersecting circles.

The following figures show the different parts of a circle. Scroll down the page for more examples and explanations.

In geometry, a circle is a closed curve formed by a set of points on a plane that are the same distance from its center O. That distance is known as the radius of the circle.

The diameter of a circle is a line segment that passes through the center of the circle and has its endpoints on the circle. All the diameters of the same circle have the same length.

A **chord** is a line segment with both endpoints on the
circle. The diameter is a special chord that passes through the center of the circle. The diameter would be the longest chord in the circle.

The radius of the circle is a line segment from the center of the circle to a point on the circle. The plural of radius is radii.

In the above diagram, *O* is the center of the circle and and are radii of the circle. The radii of a circle are all the same length. The radius is half the length of the diameter.

An arc is a part of a circle.

In the diagram above, the part of the circle from B to C forms an arc.

An arc can be measured in degrees.

In the circle above, arc BC is equal to the ∠BOC that is 45°.

A tangent is a line that touches a circle at only one point. A tangent is perpendicular to the radius at the point of contact. The point of tangency is where a tangent line touches the circle.

In the above diagram, the line containing the points B and C is a tangent to the circle.

It touches the circle at point B and is perpendicular to the radius . Point B is called the point of tangency.

is perpendicular to i.e.

A

A

A

Two circles that do not intersect can either have a common external tangent or common internal tangent.

In the

In the

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