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.
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°.
● Angle.
Interior and Exterior of an Angle.
Measuring an Angle by a Protractor.
Construction of Angles by using Compass.
Geometry Practice Test on angles.
5th Grade Geometry Page
5th Grade Math Problems
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