Students can go through AP Board 9th Class Maths Notes Chapter 12 Circles to understand and remember the concepts easily.

## AP State Board Syllabus 9th Class Maths Notes Chapter 12 Circles

→ A collection of all points in a plane which are at a fixed distance from a fixed point in the sapie plane is called a circle. The fixed point is called the centre and the fixed distance is called the radius of the circle.

→ A line segment joining any two points on the circle is called a chord.

→ The longest of all chords which passes through the centre is called a diameter.

→ Circles with same radii are called congruent circles.

→ Circles with same centre and different radii are called concentric circles.

→ Diameter of a circle divides it into two semi-circles.

→ The part between any two points on the circle is called an arc.

→ The area enclosed by a chord and an arc is called a segment. If the arc is a minor arc then it is called the minor segment and if the arc is major arc then it is called the major segment.

→ The area enclosed by an arc and the two radii joining the end points of the arc with centre is called a sector.

→ Equal chords of a circle subtend equal angles at the centre.

→ Angles in the same segment are equal.

→ An angle in a semi circle is a right angle.

→ If the angles subtended by two chords at the centre are equal, then the chords are congruent.

→ The perpendicular from the centre of a circle to a chord bisects the chords. The converse is also true.

→ There is exactly one circle that passes through three non-collinear points.

→ The circle passing through the three vertices of a triangle is called a circumcircle.

→ Equal chords are at equal distance from the centre of the circle, conversely chords at equidistant from the centre of the circle are equal in length.

→ Angle subtended by an arc at the centre of the circle is twice the angle subtended by it at any other point on the circle.

→ If the angle subtended by an arc at a point on the remaining part of the circle is 90°, then the arc is a semi circle.

→ If a line segment joining two points subtends same angles at two other points lying on the same side of the line segment, the four points lie on the circle.

→ The sum of pairs of opposite angles of a cyclic quadrilateral are supplementary.