Basic Concepts of a Circles and its Related Terms - Circles, Class 9, Mathematics PDF Download

CIRCLE

A circle is the locus of a point which moves in a plane in such a way that its distance from a fixed point remains constant. The fixed point is called the centre and the constant distance is

called the radius of the circle. The given figure consists of a circle with centre O and radius equal to r units.

TERMS AND FACTS RELATED TO CIRCLES

Radius : A line segment joining the centre and a point on the circle is called its radius, generally denoted by r. The plural of radius is radii.
In the figure, OA, OB and OC are the radii of a circle.

Circumference : The perimeter of a circle is called its circumference. O

Circumference = 2πr
Position of a Point With Respect To a Circle Let us consider a circle with centre O and radius r. A point P is said to lie.
(i) inside the circle, if OP < r. (ii) on the circle, if OP = r.
(iii) outside the circle, if OP > r. O r P

In the figure of a circle with centre O and radius r, Z
(i) The points A, O, B lie inside the circle; (ii) The points P, Q, R lie on the circle;
(iii) The points X, Y, Z lie outside the circle.

Interior and Exterior of a Circle

The region consisting of all those points which lie inside a circle,

is called the interior of the circle.
The region consisting of all those points which lie outside a circle, is called the exterior of the circle.

Circular Region or Circular Disc
The region consisting of all those points which are either on the circle or lie inside the circle, is called the circular region.

Chord : A line segment joining any two points on a circle is called a chord of the circle. In the figure, PQ, RS and AB are the chords of a circle with centre O. O

Diameter : A chord of the circle passing through the centre of a circle is called its diameter. In the figure, AOB is a diameter of a circle with centre O.

Diameter = 2 × Radius

Properties :(i) Diameter is the largest chord of a circle.
(ii) All diameters of a circle are equal in length.

Secant : A line which intersects a circle in two distinct points is called a secant of the circle. In the figure, the line cuts the circle in two points C and D. So, is a secant of the circle.

Tangent : A line that intersects the circle in exactly one point is called a tangent of the circle. The point at which the tangent intersects the circle is called its point of contact. In the figure, SPT is a tangent at the point P of the circle with centre O. Clearly, P is the point of contact of the tangent with the circle. O

Facts About Tangents :
(i) No tangent can be drawn to a circle through a point inside the circle:
(ii) One and only one tangent can be drawn to a circle at a point on the circle.
(iii) Two tangents can be drawn to a circle from a point outside it.
In the adjoining figure, PT₁ and PT₂ are the tangents to the circle from point P.

Touching Circles : Two circles are said to touch each other if and only if they have one and only one point in common. Two circles may touch externally [Fig. (i)] or internally T T [Fig. (ii)].


The common point is called the point of contact, and the line joining their centres is called the line of centres. A line touching the two circles is called a common tangent. Thus, in the above figure, P is the point of contact, AB is the line of centres and PT is a common tangent.

Direct Common Tangents: A common tangent to two circles is called a direct common tangent if both the circles lie on the same side of it. In the figure, AB and CD are two direct common tangents.

Transverse Common Tangents : A common tangent to two circles is called a transverse common tangent if the circles lie on its opposite sides. In the figure, PQ and RS are two transverse common tangents.

Arc : A continuous piece of a circle is called an arc of the circle. Let P and Q be any two points on a circle with centre O. Then, clearly the whole circle has been divided into two pieces, namely arc PAQ and arc QBP, to be denoted by ∠PAQ and ∠QBP respectively. P Q

We may denote them by ∠PQ and∠QP respectively.

Minor and Major Arc : An arc less than one-half of the whole arc of a circle is called a minor arc, and an arc greater than one-half of the whole arc of a circle is called a major arc of the circle.
Thus, in the above figure, ∠PQ is a minor arc, while ∠QP is a major arc.

Central Angle : An angle subtended by an arc at the centre of a circle is called its central angle. In the given figure, central angle of ∠PQ = ∠POQ.

Degree Measure of An Arc : Let ∠PQ be an arc of a circle with centre O.
If ∠POQ = θ°, we say that the degree measure of ∠PQ is θ° and we write, m(∠PQ ) = θ°. If m(∠PQ ) = θ°, then m(∠QP ) = (360 – θ)°. Degree measure of a circle is 360°.

Congruent Arcs : Two arcs AB and CD are said to be congruent, if they have same degree measure.

∠AB  ≌ ∠ CD ⇔ m(∠AB ) ⇔ m(∠CD ) ⇔ ∠AOB = ∠COD.

Semi-Circle : A diameter divides a circle into two equal arcs. Each of these two arcs is called a semi-circle. The degree measure of a semi-circle is 180°. In the given figure of a circle with centre O, ∠ABC as well as ∠ADC is a semi-circle.

Congruent Circles : Two circles of equal radii are said to be congruent.

Concentric Circles : Circles having same centre but different radii are called concentric circles.

Concyclic Points : The points, which lie on the circumference of the same circle, are called concyclic points.
In the adjoining figure, points A, B, C and D lie on the same circle and hence, they are concyclic.

Segment : A segment is a part of a circular region bounded by an arc and a chord, including the arc and the chord. The segment containing the minor arc is called a minor segment, while the other one is a major segment. The centre of the circle lies in the major segment.

Alternate Segments of a Circle : The minor and major segments of a circle are called alternate segments of each other.

Sector of a Circle : The part of the plane region enclosed by an arc of a circle and its two bounding radii is called a sector of the circle.
Thus, the region OABO is the sector of a circle with centre O.

Quadrant : One-fourth of a circular disc is called a quadrant.

Cyclic Quadrilateral : If all the four vertices of a quadrilateral lie on a circle, then such a quadrilateral is called a cyclic quadrilateral.

If four points lie on a circle, they are said to be concyclic.
We also say that quad. ABCD is inscribed in a circle with centre O.

Theorem : Equal chords of a circle subtend equal angles at the centre.
Given : A circle with centre O in which chord PQ = chords RS.

To prove : ∠POQ = ∠ROS
Proof : In ΔPOQ and ΔROS,

Hence proved