**Important Concepts and Formulas**

3. Closed curves have the same starting
and ending point.

4. An angle is formed by two rays
starting from a common end point.

5. Angle AOB is written as ∠AOB. Its vertex O is always written in the middle. OA and OB
are called the arms of ∠AOB or ∠BOA.

7. The meeting point of a pair of sides
is called a vertex.

8. Any two sides of a polygon having a common
endpoint are called adjacent sides.

10. A triangle is a polygon having least
number of sides.

11. A quadrilateral has four sides, four
angles and four vertices.

14. Diameter is the longest chord in a
circle.

16. The region in the interior of a
circle enclosed by an arc and a pair of radii is called a sector.

18. A radius and a diameter of a circle
are related to each other as follows.

Diameter = 2 × radius.

Diameter = 2 × radius.