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.

(i) Parallel lines. Lines which do not intersect anywhere are called parallel lines.

(ii)    Perpendicular lines. Two lines which are at a right angle to each other are called perpendicular lines.

(iii)    Line segment. It is a terminated line.

(iv)    Radius. The length of the line-segment joining the centre of a circle to any point on its circumference is called its radius.

(v)    Square. A quadrilateral with all the four sides equal and all the four angles of measure 90° each is called a square.