# NCERT Class 9 Solutions: Introduction to Euclid՚s Geometry (Chapter 5) Exercise 5.1 – Part 2 (For CBSE, ICSE, IAS, NET, NRA 2022)

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Q-2 Give a definition for each of the following terms. Are there other terms that need to be defined first? What are they how might you define them?

- parallel lines
- perpendicular lines
- line segment
- radius of a circle
- square

Solution:

To define the terms given in the question we need to define the following terms first

**Point**:- A small dot made by sharp pencil on a sheet paper gives an idea about a point. A point has no dimension, it has only position. Ideal point is not possible.

**Line**:- A line is the set of points which has length only and no breadth. The basic concept about a line is that it should be straight and that it should extend in definitely in both the directions.

**Plane**:- The surface of a smooth wall or the surfaces of a sheet of paper are close examples of a plane.

**Ray**:- A part of line l which has only one end-point A and contains the point B is called a ray AB.

**Angle**:- An angle is the union o two non-collinear rays with a common initial point.

**Circle**:- A circle is the set of all those points in a plane whose distance from a fixed point remains constant. The fixed point is called the centre of the circle.

OA = OB = OC = radius

**Quadrilateral**:- A closed figure made of four line segment is called a quadrilateral

**Parallel Lines**:- Line which do not intersect each other anywhere are called parallel lines.

In fig m and n parallel.

**Perpendicular lines**:- Two lines AB and CD lying the same plane are said to be perpendicular, if they form a right angle. We write .

**Line-segment**:- A line-segment is a part of line. When two distinct points, say A and B on a line are given, then the part of this line with end-points A and B is called the line-segment.

It is named as AB, AB and BA denote the same line-segment.

**Radius**:- The distance from the centre to a point on the circle is called the radius of the circle. In the following figure OP is the radius.

**Square**:- A quadrilateral in which all the four angles are right angles and four sides are equal is called a square. ABCD is a square.