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Obtuse angle: An angle greater than 90° but less than 180° is called A
an obtuse angle. For example, 100°, 130°, 185°, etc. 130°

O B
Straight angle: An angle equal to 180° is called a straight angle. 180°

A O B

Reflex angle: An angle greater than 180º but less than 360° is called A
a reflex angle. For example, 190°, 230°, 305°, etc.

230° O B

Complete angle: An angle equal to 360° is called a complete angle. B
O
360° A

Zero angle: If the initial position and the final position of rotating ray
coincide without making any revolution, then it is called zero angle. O B A

Related Angles

Angles that are related to each other based on certain conditions are called related angles. These
are the pair of angles related to each other with specific names.
Complementary Angles

Two angles are called complementary angles, if the sum of the measures of these angles is 90°.
Each of the angles is called complement of the other.
Following are some examples of complementary angles.

35° 65° 53°
55° 25° 37°

(i) (ii) (iii)
In fig (i), sum of two angles = 35° + 55° = 90°
In fig (ii), sum of two angles = 65° + 25° = 90°

In fig (iii), sum of two angles = 53° + 37° = 90°
Example 1: Find the complement of the following angles.
1 °
(a) 68° (b) 47° (c) 83° (d) 37
2
Solution: Two angles are said to be complementary angles if the sum of these angles is 90°.

(a) Complement of 68° = 90° – 68° = 22°
(b) Complement of 47° = 90° – 47° = 43°
(c) Complement of 83° = 90° – 83° = 7°
1 ° 1 ° 1 °
°
(d) Complement of 37 = 90 − 37 = 52
2 2 2

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