Page 9 - Cambridge IGCSE Mathematics Core and Extended

Example 1
TIPS
(a) Name the polygon on the right.
(b) Find • A triangle does not have
(i) the number of vertices, any diagonal.
(ii) the number of diagonals • We usually use capital
of the polygon. letters to label the vertices
of a polygon.
Solution
(a) Octagon The polygon has 8 sides. 1
8 2
(b) (i) Number of vertices = Number of sides
= 8 7 3
n × (n – 3)
(ii) Number of diagonals = 6 4
2 5
8 × (8 – 3)
=
2
8 × 5
=
2
= 20

Test Yourself 11.1

1. Name each of the following polygons. 2. Determine the number of sides, vertices and
(a) (b) diagonals in each of the following polygons.
(a) (b)

(c) (d)
(c) (d)

11.2 Symmetry

A. Determining and drawing line(s) of symmetry

A shape is said to have symmetry if it has two parts that overlap each other perfectly when folded along
a line. This line is known as the line of symmetry or the axis of symmetry.
For example,
Lines of
symmetry

Line of
symmetry
(a) (b)

Diagram (a) has one line of symmetry whereas Diagram (b) has two lines of symmetry.

Chapter 11 Polygons I 149

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DA1501_pg 147 to 161.indd 149
DA1501_pg 147 to 161.indd 149 14/07/2022 10:14 AM