86 MATHEMATICS
File Name : C:\Computer Station\Maths-IX\Chapter\Chap-5\Chap-5 (02–01–2006) PM65
- If a point C lies between two points A and B such that AC = BC, then prove that
AC =1
2
AB. Explain by drawing the figure.- In Question 4, point C is called a mid-point of line segment AB. Prove that every line
segment has one and only one mid-point. - In Fig. 5.10, if AC = BD, then prove that AB = CD.
Fig. 5.10- Why is Axiom 5, in the list of Euclid’s axioms, considered a ‘universal truth’? (Note that
the question is not about the fifth postulate.)
5.3 Equivalent Versions of Euclid’s Fifth Postulate
Euclid’s fifth postulate is very significant in the history of mathematics. Recall it again
from Section 5.2. We see that by implication, no intersection of lines will take place
when the sum of the measures of the interior angles on the same side of the falling line
is exactly 180°. There are several equivalent versions of this postulate. One of them is
‘Playfair’s Axiom’ (given by a Scottish mathematician John Playfair in 1729), as stated
below:
‘For every line l and for every point P not lying on l, there exists a unique line
m passing through P and parallel to l’.
From Fig. 5.11, you can see that of all the lines passing through the point P, only line
m is parallel to line l.
Fig. 5.11
This result can also be stated in the following form:
Two distinct intersecting lines cannot be parallel to the same line.