# NCERT Class 9 Solutions: Introduction To Euclid's Geometry (Chapter 5) Exercise 5.2

Euclid’s 5^{th} postulates

Euclid’s fifth postulates states that If the sum of the interior angles α and β is less than 180°, the two straight lines, produced indefinitely, meet on that side

Q-1 How would you rewrite Euclid’s fifth postulate so that it would be easier to understand?

Solution:

First lets understand the Euclid’s 5^{th} postulate. It is equivalent to what is known as the parallel postulate. In the below figure , thus the lines intersect on that side.

On the other hand:

The two angles *a* and *b* are known as consecutive interior angles. If the sum of these consecutive interior angles is , then the lines do not intersect i.e. they are parallel. Thus Euclid’s 5^{th} postulate can be written as “When two parallel lines intersect a third line, then the consecutive interior angles are supplementary.”

Q-2 Does Euclid’s fifth postulate implies the existence of parallel lines? Explain.

Solution:

As we understood in the previous question, when the two lines (*m* and *n* above) intersect the third line (*l*). Then, if the sum of angle 4 and 3 above is less than , then lines will intersect on side A. If the sum of angle 1 and 2 above is less than , then lines will intersect on side B. If both and , than lines *m* and *n* will be parallel and will not intersect on either side.