Page 93 - Maths Skill - 6

Lines and Angles 91

Right Angle
An angle which measures 90° is called a right angle. The arms of a right angle are
perpendicular to each other. In Fig. (b), ∠AOB is a right angle.
Right Angle Tester

(i) Take a piece of paper.
(ii) Fold it in somewhere of middle. Fig. (b)
(iii) Fold again the straight edge your tester is ready.

Suppose any shape with corner is given. You can use your right angle tester to test the angle of the shape.

Straight Angle
An angle which measures 180° is called a straight angle. In Fig. (c), ∠AOB is a
straight angle. The arms of a straight angle extend in opposite directions.
Fig. (c)

Complete Angle

An angle which measures 360° is called a complete angle. In Fig. (d), ∠AOB is a
complete angle. Fig. (d)

Acute Angle
An angle which measures greater than 0° but less than 90° is called an acute angle.

In Fig. (e), ∠AOB is an acute angle.

Fig. (e)
Obtuse Angle

An angle which measures greater than 90° but less than 180° is called an obtuse
angle.

In Fig. (f), ∠AOB is an obtuse angle.

Fig. (f)
Reflex Angle
An angle which measures greater than 180° but less than 360° is called a reflex angle.

In Fig. (g), ∠AOB is a reflex angle.

Fig. (g)

Let’s Attempt

Example: Write the degrees in two right angles.
Solution: 1 right angle = 90°
\ 2 right angles = 2 × 90° = 180°
Hence, the degrees in two right angles = 180°.