Give a definition for each of the following terms. Are there other terms that need to be defined first? What are they, and how might you define them?
(i) parallel lines 

(ii) perpendicular lines 

(iii) line segment

(iv) radius of a circle 

(v) square

We have to define ‘Ray’, ‘Straight line’ and a ‘point’.

Ray: A part of a line, which starts at a point and goes off in a particular direction to infinity possibly through a second point.

Straight line: The basic concept about a line is that it should be straight, and it can be extended infinitely in both directions.

Point: A small dot made by a sharp pencil on a sheet of paper gives an idea about a point.

i) Parallel lines:

• If the perpendicular distance between two lines is always constant, then these are called parallel.
• If the lengths of the common perpendiculars at different points on the lines are the same, then these lines are called parallel.
• In other words, the lines which never intersect each other are called parallel lines.

ii) Perpendicular lines: If the angle between two lines is equal to 90º, then these lines are perpendicular to each other.

iii) Line segment: A terminated line is called a line segment. It has two endpoints.

iv) Radius of a circle: The distance from the center to any point on the circle is called the radius of the circle.

v) Square: A square is a regular quadrilateral which means that it has four equal sides and four right angles.