ANGLE

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ANGLE.‎

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ANGLE

An angle is a figure formed by two rays with same initial point.
The two rays forming an angle are called the ‘arms’ (or the sides or the legs) of the angle and the common initial (end) point is called the ‘vertex’ of the angle.
An angle looks like the one in figure. Here, A is the vertex and rays AB and AC are the arms of the angle.

Representation and Naming of an Angle

(i) Two hands of a clock form an angle.
(ii) Two pages of your notebook make an angle (as shown in fig.).

Measurement of Angles
Generally, we use protractor to measure angles. In measuring angle, we use the scale with 0° on one of the arms of the angle.
In fig. below

Types of Angles
I. (i) Zero Angle : If the initial and the final positions of a ray coincide without making a revolution, then the angle formed is a zero angle.

(ii) Right Angle : The angle formed at the comer of a rectangular blackboard is a right angle. Measure of a right angle is 90°.

(iii) Straight Angle : An angle formed by two opposite rays is called a straight angle. Measure o f a straight angle is 180°.

In the adjoining figure, the angle formed by two opposite rays OA and OB is a straight angle.

(iv) Complete Angle : If a rotating ray, after completing one rotation, coincides with the initial position, then the angle formed is called a complete angle. Measure of a complete angle is 360°.

In the adjoining figure, arms OA and OB coincide after making a complete revolution.

II. (i) Acute Angle : An angle which is greater than 0° but less than 90° is known as an acute angle.

(ii) Obtuse Angle : An angle which is greater than a right angle (i.e.,90°) but less than a straight angle (i.e., 180°) is called an obtuse angle.