## Chapter 10 - Circles - Important Terms You Should Know

**circle**is a collection (set) of all those points in a plane, each one of which is at a constant distance from a fixed point in the plane.

2. The fixed point is called the

**centre**and the constant distance is called the

**radius**of the circle.

3. All the points lying inside a circle are called its

**interior points**and all those points which lie outside the circle are called its

**exterior points**.

4. The collection (set) of all interior points of a circle is called the

**interior of the circle**while the collection of all exterior points of a circle is called the

**exterior of the circle**.

5. A line segment joining two points on a circle is called the

**chord**of the circle.

6. A chord passing through the center of the circle is called a

**diameter**of the circle.

7. A line which meets a circle in two points is called a

**secant**of arcthe circle.

8. A polygon is a closed figure made up of three or more line segments (sides) such that each line segment intersects exactly two others at its end – points (vertices) and no two line segments which intersect are

**collinear**.

9. A polygon is called a

**regular polygon**, if it has all its sides equal and has all its angles equal.

10. A (continuous) part of a circle is called an

**arc**of the circle. The arc of a circle is denoted by the symbol ‘

**⌢**’.

11.

**Circumference**: The whole arc of a circle is called the circumference of the circle.

12.

**Semi-circle**: One – half of the whole arc of a circle is called a semi – circle of the circle.

13. Minor and Major arcs: An arc less than one - half of the whole arc of a circle is called a

**minor arc**of the circle, and an arc greater than one – half of the whole arc of a circle is called a

**major arc**of the circle.

14. Central Angle: Any angle whose vertex is centre of the circle is called a

**central angle**.

15. Degree measure of an Arc: The

**degree measure of a minor arc**is the measure of the central angle subtended by the arc.

16.

**Congruent Circles**: Two circles are said to be congruent if and only if either of them can be superposed on the other so as to cover it exactly.

17.

**Congruent Arcs**: Two arcs of a circle (or of congruent) circles) are congruent if either of them can be superposed on the other so as to cover it exactly.

18.

**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 a circle.

19.

**Segment of a circle**: A chord of a circle divides it into two parts. Each part is called a segment.

20. The part containing the minor arc is called the

**minor segment**, and the part containing the major arc is called the

**major segment**.

21. A quadrilateral, all the four vertices of which lie on a circle is called a

**cyclic quadrilateral**. The four vertices A, B, C and D are said to be

**Concyclic**points.