# Measuring an Angle by a Protractor

In measuring an angle by a protractor, first we need to know what a protractor is.

Protractor:

We measure the size of an angle by using protractor. The protractor is in the shape of a semicircle. The semicircle is divided into 180 equal parts which show angle measurements from 0° to 180°.

The protractor has two sets of measurements. The marking on the inner circle is 0° to 180° in the opposite direction to the markings on the outer circle.

It is an instrument for measuring or constructing an angle of a given measure. It is a circular or semicircular piece of metal or plastic.

### It is a semicircular protractor which is marked in degrees from 0° to 180° from left as shown in adjoining figure.

To measure an angle ABC, we place the mid point of the protractor on the vertex O of the angle. The base OA arm falls along with the base line of the protractor as shown in the figure below.

The angle is measures on the protractor counting from 0 up to the point where the arm OB lies. In the above figure ∠AOB = 60°.

The angle which is formed outside is known as the exterior angle.

In the above figure ∠2 is the exterior angle and ∠1 is known as interior angle.

For Example:

1. Use your protractor to draw 60°.

The centre O of the piece is also the midpoint of its base line. In order to measure ∠AOB, place the protractor in such a way that its centre is exactly on the vertex O of the angle, the base line lies along the arm OA. We need to read the mark through which the arm OB passes, starting from O on the side A, as we observe in the above figure.

Thus we find ∠ AOB = 60°.

2. Use your protractor to draw 90°.

The centre O of the piece is also the midpoint of its base line. In order to measure ∠AOB, place the protractor in such a way that its centre is exactly on the vertex O of the angle, the base line lies along the arm OA. We need to read the mark through which the arm OB passes, starting from O on the side A, as we observe in the above figure.

Thus we find ∠ AOB = 90°.

Interior and Exterior of an Angle.

Measuring an Angle by a Protractor.

Types of Angles.

Pairs of Angles.

Bisecting an angle.

Construction of Angles by using Compass.

Worksheet on Angles.

Geometry Practice Test on angles.

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