Page 103 - Start Up Mathematics_6

Classification of Triangles
Triangles are classified in the following two ways:
1. According to their sides

2. According to their angles
A
Classification of triangles according to their sides:

(i) Scalene triangle: A triangle in which all the AB ≠ BC ≠ CA
sides are of different length is known as a
scalene triangle.

B C
A

(ii) Isosceles triangle: A triangle having two
equal sides is known as an isosceles triangle. AB = CA

B C

A

(iii) Equilateral triangle: A triangle having all AB = BC = CA
three sides equal is known as an equilateral
triangle.
B C
A
Classification of triangles according to their angles:

(i) Acute-angled triangle: A triangle having all acute angles
is known as an acute-angled triangle or an acute triangle.
In the adjoining figure, ∆ ABC is an acute-angled triangle B C
as each of the angles ∠A, ∠B and ∠C are less than 90°. A

(ii) Right-angled triangle: A triangle having one of its angles
as right angle is known as a right-angled triangle or a
right triangle. In the adjoining figure, ∆ ABC is right- B C
angled as ∠ABC = 90°.
A
(iii) Obtuse-angled triangle: A triangle having one of its
angles as obtuse angle is known as an obtuse-angled
triangle or an obtuse triangle. In the adjoining figure,
∆ ABC is obtuse-angled as ∠ABC > 90°.

Example 18: Match the following: B C
(a) 3 sides of equal length (i) Scalene
(b) 2 sides of equal length (ii) Right-angled isosceles
(c) All sides of different lengths (iii) Obtuse-angled

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