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.

**Related Pages**

Circles

Tangent Of A Circle

Chords Of A Circle

The following figures show the different parts of a circle: tangent, chord, radius, diameter, minor arc, major arc, minor segment, major segment, minor sector, major sector. 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.The following video gives the definitions of a circle, a radius, a chord, a diameter, secant, secant line, tangent, congruent circles, concentric circles, and intersecting circles.

A **secant line** intersects the circle in two points.

A **tangent** is a line that intersects the circle at one point.

A **point of tangency** is where a tangent line touches or intersects the circle.

**Congruent circles** are circles that have the same radius but different centers.

**Concentric circles** are two circles that have the same center, but a different radii.

**Intersecting Circles**: Two circles may intersect at two points or at one point. If they
intersect at one point then they can either be externally tangent or internally tangent.

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

In the **common external tangent**, the tangent does not cross between the two circles.

In the **common internal tangent**, the tangent crosses between the two circles.

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