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Measuring an angle

Measuring an angle involves the use an instrument called the protractor (see picture below). It has two scales marked on it – an inner scale and an outer scale. The inner scale is used to measure the angle in an ‘anti-clockwise’ direction, and the outer scale is used to measure the angle in a ‘clockwise’ direction.

The unit of measurement for an angle is degrees, and is represented by a small circle next to the number (like 20°).

Protractor

To actually measure the size of an angle, we follow the following steps:

Step 1: Place the central point of the protractor on the vertex of the angle.

Step 2: Line up the base line of the protractor with the lower ray (first ray or arm) or the angle.

Step 3: Measure the angle from the base line using the scale which begins at 0° until you reach the second ray/arm.

Step 4: The position of the second ray/arm will will the size of the angle on the protractor.

Tip: If the arms are too short to read/measure the angle, make it longer with a pencil and ruler before measuring.

angle measurement of 50

 

The angle of 50º is measured on inner scale, when angle is measured anticlockwise.

measurement of angle 120

An angle measurement of 120º on outer scale is measured, when angle is measured clockwise.