Maths NCERT Solutions Class 9 – Chapter 11 – Circle – Part 1
Class 9 Maths Circle – A circle is the locus of a point which moves in a plane in such a way that its distance from a given fixed point is always constant.
The permanent tip is called the centre and the stable distance is called the radius of the circle.
A circle with centre O and radius r is denoted by C (O,r).
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Terms Related to a Circle
Radius : A Line segment joining the centre and a point on the circle is called its radius.
The Plural of Radius is radii.
In the given figure, OA and OB are radii of circle C (O, r)
Circumference : The perimeter of a circle is called its circumference.
Circumference = 2πr.
Chord-A chord of a circle is a line segment joining any two points on the circle.
In the given figure, PQ, RS, and AOB are the chords of a circle with centre O.
Diameter-A diameter is a chord of a circle passing through the centre of the circle.
Hence AOB is a diameter of a circle with O.
A diameter is the longest chord of a circle.
Secant-A line which intersects a circle in two distinct points is called a scent of the circle.
In the given figure, the line l cuts the circle in two points A and B. So l is a secant of the circle.
Tangent-A line that intersects the circle in exactly one point is called a tangent to the circle. The point at which the tangent meets the circle is called its point of contact.
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Position of a point with respect to a circle
A point P is said to lie
- Inside the circle C (O,r), if OP<r
- On the circle C(O,r)if OP=r
- Outside the circle C(O,r)if OP>r
Interior of a circle-The region consisting of all points lying on the circumference of a circle and inside it is called interior of a circle. A point P lies in the interior of a circle C (O,r) if OP<=r.
The exterior of a circle-The region consisting of all points lying outside a circle is called exterior of a circle. A point P lies in the exterior of a circle C (O,r) if OP>r.
The circular region-The region consisting of all points which are either on the circle or lie inside the circle is called the circular region or circular disc.
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