Page 228 - Math Smart - 7

In a regular polygon all sides are of the same length and all angles are of the
same size.

equilateral square regular regular regular
triangle pentagon hexagon octagon
Let us look closely at each type of polygons.

Triangles

We can classify triangles in two ways. Let us recall some.

1. Grouping according to the lengths of their sides

• Triangle A has 3 equal sides.
It is an equilateral triangle.
• Triangles D and E each have 2 equal sides.
They are Isosceles triangles.
• Triangles B, C and F do not have any equal sides.
They are scalene triangles.

Grouping according to the types of angles
• The angles in triangles A and D are all acute angles.
They are acute-angled triangles.
• Observe that triangles C and E each have an obtuse angle.
They are obtuse-angled triangles.
• Observe that triangles B and F each have a right angle.
They are right-angled triangles.

Example 1

O Group the triangles according to:
a) the lengths of their sides and
b) the types of angles.

Solution
a)

Type of triangle Triangle Type of triangle Triangle
Equilateral A Acute-angled A, E, G

Isosceles E, F Obtuse-angled B, C
Scalene B, C, D, G Right-angled D, F

B UNIT 10 I Shapes and Symmetry