Page 165 - ICSE Math 6

Points to remember

• As the two rays forming an angle extend endlessly, therefore the interior and exterior of the angle
also extend endlessly.
• The arms of an angle separate its interior and exterior.

Magnitude of an angle
Every angle has a size or magnitude. The magnitude of an angle is the
amount of rotation which one of the arms has to take around the vertex S P
to coincide with the other arm. In the adjoining figure, the magnitude of
∠PQR is the amount of rotation required by arm QR to coincide with arm
QP. Similarly, the magnitude of ∠SQR is the amount of rotation required
by arm QR to coincide with arm QS. The unit for measuring an angle is Q R
degree which is denoted by ‘ ° ’.

Types of angles

On the basis of the magnitudes, angles are divided into five categories.

Acute angle
A
An angle whose magnitude is more than 0° but less than 90° is called
an acute angle. For example, 15°, 30°, 45°, etc. Acute angle
O B

Right angle A
Right angle
An angle whose magnitude is 90° is known as a right angle.
90°
O B
Obtuse angle
An angle whose magnitude is more than 90° but less than 180° is A
called an obtuse angle. For example, 96°, 118°, 175°, etc. Obtuse angle

O B
Straight angle

An angle whose magnitude is 180° is known as a straight angle. Straight angle
A O B
Reflex angle

An angle whose magnitude is more than 180° but less than 360° is O Reflex angle
known as a reflex angle. For example, 188°, 220°, 350°, etc. B

A
Points to remember
• Two lines or rays or line segments that form a right angle are B A
perpendicular to each other.

• Sum of angles around a point is always 360°. For example, in the O D
adjoining figure, C

∠ AOB + ∠BOC + ∠COD + ∠DOA = 360°.

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