We are given that,
AC = BC 
Also, point C lies between points A and B.
Adding AC to both sides:
\(AC + AC = BC + AC\) ... (1)
Here, (BC + AC) coincides with AB. This is based on the property that things which coincide with one another are equal to each other. Therefore, we have:
\(BC + AC = AB\) ... (2)
We also know the property that things which are equal to the same thing are equal to each other. Hence, from equations (1) and (2), we can deduce the following:
\(AC + AC = AB\)
This simplifies to:
\(2AC = AB\) ... (3)
Finally, dividing both sides of equation (3) by 2, we get:
\(AC = \frac{1}{2} AB\)
This concludes the proof that the length of AC is half the length of AB.
In Fig. 5.10, if AC = BD, then prove that AB = CD.

Consider two ‘postulates’ given below :
(i) Given any two distinct points A and B, there exists a third point C which is in between A and B.
(ii) There exist at least three points that are not on the same line.
Do these postulates contain any undefined terms? Are these postulates consistent?
Do they follow from Euclid’s postulates? Explain.