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.
Which of the following statements are true and which are false? Give reasons for your answers.
(i) Only one line can pass through a single point.
(ii) There are an infinite number of lines which pass through two distinct points.
(iii) A terminated line can be produced indefinitely on both the sides.
(iv) If two circles are equal, then their radii are equal.
(v) In Fig., if AB = PQ and PQ = XY, then AB = XY

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
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.