Basic Terms and Definitions

**(i) Line segment:** A part of a line with two endpoints is called a line segment.

Line segment AB is denoted by .

**(ii) Ray:** A part of a line with one endpoint is called a ray.

The ray AB is denoted by .

**(iii) Collinear points and non-collinear points:** If three or more than three points he on the same line, then they are called collinear points, otherwise, they are non-collinear points.

P, Q and R are collinear points.

A, B and C are non-collinear points.

A, B and C are non-collinear points.

2. **Angle:** An angle is formed when two rays originate from the same endpoint.

Angle ABC is denoted by ∠ABC

The rays ( and ) making an angle are called the arms of ∠ABC.

The end point (B) is called the vertex of ∠ABC.

**Types of Angles:** There are different types of angles such as acute angle, right angle, obtuse angle, straight angle and reflex angle.

(i) **Acute angle:** An acute angle is an angle which is less than 90°.

Acute angle : 0° < x < 90°.

(ii) **Right angle:** A right angle is an angle which is equal to 90°.

Right angle : y = 90°

(iii) **Obtuse angle:** An obtuse angle is an angle which is more than 90° and less than 180°.

Obtuse angle : 90° < z < 180°

(iv) **Straight angle:** A straight angle is an angle which is equal to 180°.

(v) **Reflex angle:** A reflex angle is an angle, which is more than 180° and less than 360°.

Reflex angle : 180° < t < 360°

**Complementary Angles:** Two angles whose sum is 90° are called complementary angles.

**Supplementary Angles:** Two angles whose sum is 180° are called supplementary angles.

**Adjacent Angles:** Two angles are adjacent if they have a common vertex, a common arm and their non-common arms are on different sides of the common arm.

∠ABD and ∠DBC are the adjacent angles. Ray BD is their common arm and point B is their common vertex. Ray BA and ray BC are non-common arms.

**Vertically Opposite Angles:** The vertically opposite angles formed when two lines intersect each other at a point.

NCERT Solutions Ch- 6 TextBook