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Complete the following statements.

1. A closed figure bounded by three line segments is called a .............................................
2. The number of sides and the number of angles in a triangle are ....................... and .............

3. A triangle with all its sides of unequal lengths is known as .............................................
4. A triangle will all three equal sides is called a/an .............................................
5. A triangle having one angle measuring 90° is called a .............................................

Triangle

You have learnt in the earlier classes that ‘a simple closed plane figure bounded by three line
segments is called a triangle or a 3-sided polygon is called a triangle.

The three line segments are called its sides and each point where two line segments meet is called
its vertex.
In the adjoining figure, ABC is a triangle whose three sides are AB, BC and CA; A
three vertices are A, B and C; and three angles are ∠A, ∠B, and ∠C or ∠BAC,
∠ABC, and ∠ACB.
A triangle is named by the names of its three vertices and is denoted by the
symbol D. We can denote the triangle by DABC or DBCA or DCAB or DCBA. B C

Types of Triangles
Triangles are classified in two ways:
• Based on the lengths of the sides.
• Based on the measures of the angles.
Classification Based on the Lengths of the Sides

Scalene triangle: A triangle whose all three sides are unequal is B
known as a scalene triangle.
In the adjoining figure, DABC is a scalene triangle as AB ≠ BC ≠ CA
In a scalene triangle, all three angles are also unequal.
That is, ∠A ≠ ∠B ≠ ∠C A C

Isosceles triangle: A triangle whose any two sides are equal is E
known as an isosceles triangle.
In the adjoining figure, DDEF is an isosceles triangle in which DE = EF

In an isosceles triangle, the angles opposite to equal sides are also
equal. That is in DDEF, ∠EDF = ∠EFD D F
Equilateral triangle: A triangle whose all three sides are of equal A
length is called an equilateral triangle. 60°
In the adjoining figure, DABC is an equilateral triangle in which
AB = BC = CA.
60° 60°
Also, the measure of each angle is 60° in equilateral triangles. B C

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