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c. An equilateral triangle is also an/a _________.

i. scalene triangle ii. acute-angled triangle
iii. right-angled triangle iv. obtuse-angled triangle

d. Which triangle can have all sides of different lengths?

i. Equilateral triangle ii. Isosceles triangle

iii. Scalene triangle iv. None of these

e. The number of obtuse angles a triangle can have is:
i. 0 ii. 1 iii. 2 iv. 3

QuADrIlATerAlS
D C
A quadrilateral is a polygon formed by four line segments.

A quadrilateral has four sides, four vertices and four angles.
Observe the quadrilateral ABCD shown alongside.

Quadrilateral ABCD has A B

i. Four vertices: A, B, C and D. Keep in Mind

ii. Four sides: AB, BC, CD and DA.
A diagonal is a line segment
iii. Four angles: ∠DAB, ∠ABC, ∠BCD, ∠CDA. that joins two non-consecutive

iv. Two diagonals: AC and BD. vertices of a polygon.
• Two sides of a quadrilateral with a common endpoint are called adjacent sides.

Here, (AB, BC); (BC, CD), (CD, DA) and (DA, AB) are the four pairs of adjacent
sides.
• Two sides of a quadrilateral are called opposite sides if they do not share a

common endpoint. Here, (AB, DC) and (AD, BC) are two pairs of opposite sides.
• Two angles of a quadrilateral that have a common side are called adjacent

angles. Here, (∠A, ∠B), (∠B, ∠C), (∠C, ∠D), and (∠D, ∠A) are four pairs of
adjacent angles.

• Two angles of a quadrilateral that are not adjacent are known as opposite
angles. Here, (∠A, ∠C) and (∠B, ∠D) are two pairs of opposite angles.
• The sum of all interior angles of a quadrilateral is 360°.

Types of Quadrilaterals
Based on the lengths of the sides and diagonals, and measures of the angles, we can

define several special types of quadrilaterals.

122 Mathematics 5