**Definition
of a Circle**

A circle is
the locus of all those points which are equidistant from a fixed point on a
given plane. The fixed point is called the

**center**of the circle.
The distance
between the center of the circle and any point on the circle is called the

**radius**of the circle. In the given figure below, OA is a radius of the circle.
A line segment
passing through the center and touching the circle at two points is called the

**diameter**of the circle.
The diameter
is twice of the radius, i.e., diameter, d = 2 × radius = 2r.

**Terms Related
to a Circle**

**Radius of a Circle**

A line segment joining the centre of a circle to any point on the circle is called the radius of the circle. In the following figure, OA is the radius of the circle. The radius of a circle is half of the diameter of the circle. Radius,

*r*=*d*/2**Diameter of a Circle**

A line segment joining any two points on the circle and passing through the centre is called the diameter of the circle. The diameter of a circle is twice the radius of the circle.

Diameter,

*d*= 2*r*

**Interior
and Exterior of a Circle **

Consider a
circle with center O and radius r. The circle divides the plane into three
parts.

1. The part of the plane consisting of
point A for which OA < r, lies in the interior of the circle.

2. The part of the plane consisting of
point B for which OB = r, lies on the circle itself.

3. The part of the plane consisting of
point C for which OC > r, lies in the exterior of the circle.

**Circular
Region **

The part of
the plane consisting of the circle and its interior is called the circular
region.

**Circumference**

The length of the boundary of the circle is called the circumference. If the radius of a circle is Ï€

*r*, then the circumference of the circle is given by 2*r*.**Chord of a circle**

A chord is a line segment whose end
points lie on the circumference. EF is a chord of the circle.

**Arcs of a
circle**

Any part of
the circumference of a circle is called an arc of the circle. The curved part
ECF is an arc of the circle.

**Major arc
and Minor arc**

A
chord that does not pass through the center divides the circumference into two
unequal arcs. The greater arc is called the major arc. The smaller arc is known
as the minor arc. In the figure, ECF is the minor arc and EAPQBF is the major
arc.

**Sectors
of a circle**

The
region encloses between two radii OP and OQ and the arc PQ is known as a sector
of the circle. The smaller region intercepted by the arc PQ is known as the
minor sector. The larger region intercepted by the arc PAEFBQ is known as the
major sector. The angle POQ is called the angle of the sector.

**Angle in
a semicircle**

It is
an angle subtended by the diameter at any point on the circumference of the
semicircle. This angle is always a right angle, i.e., 90°.

**Segments
of a circle**

The
part of the circle enclosed by an arc and the corresponding chord is called a
segment. A chord divides the region of the circle into two parts. The smaller
part is called the minor segment and the larger part is called the major
segment.

**Quadrant**

If the sector is equal to the
one-fourth of the circle, it is called the quadrant of the circle. In the given
figure, two perpendicular radii enclose the quadrant of the circle. A circle
has four quadrants.

**Secant**

A line which intersects the circle
at two distinct points is called a secant of the circle. Line AB is a secant
to the circle and line segment PQ is a chord to the circle.

**Tangent**

A line which touches the circle at
only one point is called a tangent. In the given figure, line l is the tangent
to the circle with center O. The tangent to a circle is a special case of the
secant, when two end points of its corresponding chord coincide.

**Concentric
Circles**

The circles
with the same center and different radii are called concentric circles.
Concentric circles do not intersect.