Chapter 9: Symmetry
Butterflies, rangolis, flowers and many buildings look balanced. That balance is symmetry. In this chapter we find lines that split shapes into mirror halves, and discover shapes that look the same when we turn them.
In this chapter you will
- find lines of symmetry in shapes, letters and pictures
- complete pictures to make them symmetric
- find the order and angle of rotational symmetry
Topics
- Line of Symmetry NCERT 9.1
A line of symmetry splits a shape into two halves that match exactly when folded.
- Rotational Symmetry NCERT 9.2
Rotational symmetry: the shape looks the same after less than a full turn.
Key words
- Line of symmetry
- a line that splits a shape into two matching mirror halves
- Mirror image
- what a shape looks like when reflected in a mirror
- Rotational symmetry
- looking the same after less than a full turn
- Order of rotational symmetry
- how many times a shape looks the same in one full turn
- Angle of symmetry
- the smallest turn after which a shape looks the same
Summary
- A line of symmetry divides a figure into two parts that match exactly when folded.
- A figure can have no line of symmetry, one, or many.
- A figure has rotational symmetry if it looks the same after turning less than a full turn about its centre.
- Order × angle of symmetry = 360°.