Q.2. 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
‘Point’, ‘straight line’ and ‘plane’ will have to be defined first. ‘Point’ and ‘straight line’ as defined by Euclid,
A point is that which has no part.
A line is breadthless length and a straight line is a line which lies evenly with the points on itself.
A plane surface is a surface which lies evenly with the straight lines on itself.
Right angle is an undefined term here. An angle of 90o is called right angle.
‘Line’ and ‘point’ have been defined before.
‘Centre’ may be defined (assuming inside) as a point inside the circle which is at the same distance from all points on the circle.
A quadrilateral is a figure with four sides. ‘Figure’, ‘side’ and ‘angle’ may be assumed known.