Measure angles with a protractor

Angle measurement quantifies the turn from one ray to another in degrees, from 0° to 180°, regardless of the lengths of the rays or the angle’s orientation. A protractor’s center aligns with the vertex and its baseline with one ray; the correct scale begins at 0° on that ray, distinguishing acute, right, and obtuse measures and preventing confusion between the two scales. Radian, reflex-angle, and trigonometric measurement are not included.

Detailed Explanation: Measure angles with a protractor

An angle measures how far one ray turns to reach another ray. We measure angles in degrees, written with the symbol $(^{\circ})$ .

To measure an angle with a protractor:

Place the center point of the protractor exactly on the angle’s vertex.
Line up the straight edge of the protractor with one ray.
Find the scale that starts with $(0^{\circ})$ on the ray you lined up.
Read the number where the other ray crosses the protractor.
Check whether your answer makes sense:
- Less than $(90^{\circ})$ : acute angle
- Exactly $(90^{\circ})$ : right angle
- Greater than $(90^{\circ})$ : obtuse angle

Example

Suppose we want to measure angle $(ABC)$ .

The vertex is $B$ , so place the protractor’s center on point $B$ .
Line up the straight edge with ray $(BA)$ .
Ray $(BA)$ points to the left, so use the scale whose $(0^{\circ})$ is on the left side.
Follow the other ray, $(BC)$ , until it reaches the curved edge of the protractor.
It reaches $(120^{\circ})$ .

Therefore,

m\angle ABC=120^{\circ}

Since $(120^{\circ})$ is greater than $(90^{\circ})$ but less than $(180^{\circ})$ , the angle is obtuse.

Learn by doing: Measure angles with a protractor

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Measure Angle With Protractor - Under 180 Degrees (Positive)

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