Draw $\triangle ABC$ with $AB = 6$ cm, $BC = 7$ cm and $CA = 7$ cm.
Draw the circumcircle of $\triangle ABC$. Let the circumcentre be O. Measure OA, OB, OC.                               [Page No. 98]

Construction steps:

1. Draw AC = 6 cm.

2. With B as centre and radius 6 cm, and with C as centre and radius 7 cm, draw arcs that intersect at A.
Join AB and AC.

3. Draw perpendicular bisectors of any two sides; their intersection O is the circumcentre.

4. Draw the circumcircle with centre O and radius OA.

Since, O is the circumcentre, it equidistant from all three vertices.

Therefore, OA = OB = OC = 3.9 cm (circumradius).
They must be equal because each is a radius of the circumcircle.