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Remember
Š Polygons are closed plane figures made up of only line segments which do not cross each other.
Š Each side of the polygon intersects exactly two other sides at the end points only.
Š No two sides cross each other.

TRIANGLE
A
A triangle is a closed plane figure bounded by three line segments.
It is a three sided polygon.

The symbol ‘D’ is used to represent a triangle. The triangle ABC
shown alongside is written as DABC.
A triangle has three sides, three angles and three vertices. B C

The following table tells about the parts (sides, angles, vertices) of DABC.
Part Number of Parts Name
Sides 3 AB, BC, CA

Vertices 3 A, B, C
Angles 3 ∠BAC or ∠A, ∠ABC or ∠B, ∠ACB or ∠C

Types of Triangles

We know that a triangle has three sides and three angles. Hence, triangles are classified

on the basis of their: (i) sides (ii) angles.
On the Basis of Sides

Equilateral triangle: A triangle whose all three sides are of equal length A
is called an equilateral triangle. 3
In the adjoining figure, DABC is an equilateral triangle, as AB = BC 3 cm cm
= CA = 3 cm.

Also, all the angles are equal in an equilateral triangle which are equal B 3 cm C
to 60°.
A
For example, in DABC, ∠A = ∠B = ∠C = 60°.
Isosceles triangle: A triangle whose two sides are of equal length is cm 3
called an isosceles triangle. 3 cm

In the adjoining figure, DABC is an isosceles triangle, as
AB = AC = 3 cm, and BC = 4 cm. B 4 cm C

In an isosceles triangle, the angles opposite to equal sides are also equal. In DABC, ∠B = ∠C.
Scalene triangle: A triangle whose all three sides are of different A
lengths is called a scalene triangle.
In the adjoining figure, DABC is a scalene triangle, as AB = 3 cm, 3 cm 4 cm
BC = 6 cm and CA = 4 cm. All the angles in a scalene triangle are

of different measures. B 6 cm C

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