Page 166 - ICSE Math 7

Obtuse angle

An angle whose magnitude is more than 90° but less than 180° is called P
an obtuse angle. For example, angles with magnitude 91°, 101° or 178°
are obtuse angles. Q R

Straight angle

An angle whose magnitude is 180° is called a straight angle. P Q R

Reflex angle Q R
An angle whose magnitude is more than 180° but less than 360° is called
a reflex angle. For example, angles with magnitude 183°, 190° or 325°
are reflex angles. P

Some special angles
Zero angle
If the initial and final position of a ray coincides without making any rotation, Q R(P)
then the angle formed is zero angle and its magnitude is 0°.

Complete angle

If a ray, say OA makes one complete rotation and then coincides with its
initial position, then the angle formed is known as a complete angle and its O A(B)
magnitude is 360°.

Relation between two angles
Some pairs of angles have a specific name according to their relation between the magnitudes or
relation between their arms and vertices.

Complementary angles
Two angles are called complementary angles if the
sum of their magnitudes is equal to 90°. Also, each P Maths Info
angle is called the complement of the other. S Complementary and supplementary
In the adjoining figure, angles 1 and 2 are 2 1 angles need not necessarily be
adjacent angles.
complementary angles as ∠1 + ∠2 = 90°. Q R

Supplementary angles
Two angles are called supplementary angles if the sum of their magnitudes P
is equal to 180°. Also, each angle is called the supplement of the other.

In the adjoining figure, angles 1 and 2 are supplementary angles as 2 1
∠1 + ∠2 = 180°. S Q R

Adjacent angles
Two angles are called adjacent angles if: P
(a) they have a common vertex R
(b) they have a common arm 2

(c) the other arms are on the opposite sides of the common arm Q 1 S

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