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Triangles

A triangle is a closed plane figure bounded by three line segments. A
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. The

following table tells us about the parts (sides, angles, vertices) of B C
DABC.

Part number 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 can be
classified on two bases:

a. on the basis of sides b. on the basis of angles

Types of Triangles on the Basis of sides

equilateral triangle: A triangle in which all three sides are A
equal is called an equilateral triangle.
In the adjoining figure, DABC is an equilateral triangle, since 3 cm 3 cm

AB = BC = CA = 3 cm.

Equilateral triangles have all equal angles. In DABC,
∠A = ∠B = ∠C = 60°. B 3 cm C
isosceles triangle: A triangle in which two sides are equal is

called an isosceles triangle. A
In the adjoining figure, DABC is an isosceles triangle, since

AB = AC = 3 cm and BC = 4 cm. In an isosceles triangle, the 3 cm 3 cm
angles opposite to the equal sides are also equal. In DABC,
∠B is opposite to side AC and ∠C is opposite to side AB . B 4 cm C
Therefore, ∠B = ∠C.

scalene triangle: A triangle which has three unequal sides is
called a scalene triangle. A
In the adjoining figure, DABC is a scalene triangle, since 4 cm 3 cm

AB = 4 cm, BC = 6 cm and CA = 3 cm. All the angles in a
scalene triangle are of different measures. B 6 cm C

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