Let's focus on measuring and classifying angles separately. We will start by showing how to measure an angle.

How to measure an angle

Let's draw an angle using two rays.

To measure the angle we need a protractor. The middle of the base of the protractor we put at the vertex of the angle. Align the bottom ray of the angle with the ray marked $0^{\circ}$ on the protractor as seen below.

The other ray passes the curved section of the protractor and there we read the angle's measure. We see that the measure of this angle is $5 0^{\circ} .$

How to classify an angle

When you classify an angle you first have to find its measure $m ∠ A .$ When you know the measure, you can classify the angle by looking in this table.

Type	Measure
Acute	$0^{\circ} < m ∠ A < 9 0^{\circ}$
Right	$m ∠ A = 9 0^{\circ}$
Obtuse	$9 0^{\circ} < m ∠ A < 18 0^{\circ}$
Straight	$m ∠ A = 18 0^{\circ}$

Let's look at an example.

We measure the angle and find that its measure is $11 2^{\circ} .$ Since the measure of the angle is greater than $9 0^{\circ}$ and less than $18 0^{\circ},$ the angle is classified as obtuse.

Exercise