Skill: 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:

1. Place the center point of the protractor exactly on the angle’s vertex.
2. Line up the straight edge of the protractor with one ray.
3. Find the scale that starts with $(0^{\circ})$ on the ray you lined up.
4. Read the number where the other ray crosses the protractor.
5. 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.