NCERT Solutions for Class 9th Maths Chapter 5 Definitions — Think and Reflect
Book page 93 Updated on2026-09-08
Q1.
Jamuna has a circular piece of paper. She is trying to locate its centre. Amina gives her a suggestion. She follows the instructions and is thrilled to find that it works. Can you guess what Amina told her?
Answer
Amina almost certainly told her: fold the paper in half twice; the point where the two creases cross is the centre.
Here is the procedure and the reason behind each step.
Fold the circular paper so that the boundary falls exactly on itself. Open it. The crease is a line of reflection symmetry of the circle, so it is a diameter — and every diameter passes through the centre.
Fold again in a different direction so the boundary again falls on itself. This crease is a second diameter.
Two different diameters both pass through the centre, and two distinct lines meet at only one point. So their point of intersection is the centre.
Two folds that make the boundary overlap give two diameters; they cross at the centre.
Why it happens: a fold that makes the boundary overlap itself is a reflection that maps the circle onto itself. The only lines that do this are the diameters, because reflection must keep the centre fixed and the crease is the mirror line.