What are angles?

An angle happens whenever two lines, line segments, or rays cross or meet. The angle is the space between the lines and is measured in degrees.

What are acute, obtuse, right, and reflex angles?

1. Acute angles measure less than 90°.
2. Obtuse angles measure greater than 90° (but less than 180°).
3. Right angles measure exactly 90°.
4. Reflex angles measure greater than 180° (but less than 360°).

How to measure angles:

1. Put the center of your protractor on the angle vertex (or corner).
2. Put the baseline of your protractor right over one of the angle arms.
3. Read where the other angle arm crosses the protractor.

Example Problems:

1. Classify the following angles:

2. Measure the following angle:

Don’t make this protractor mistake:

If your protractor has 2 sets of numbers (both from 0-180), it’s because you can measure angles from the left side of the protractor and the right. When reading where your second angle arm crosses the protractor, if you’re unsure whether you’re using the correct set of numbers, follow these steps:

1. Follow your chosen set of numbers back to zero.
2. Ask yourself, “Is my first angle arm pointed at this zero?”
   1. If the answer is yes, then you’ve chosen the correct set of numbers.
   2. If the answer is no, then you’ve chosen the incorrect set of numbers.

A second check is asking yourself whether your angle is acute or obtuse. If it’s obvious that your angle is acute, then the angle measure should be less than 90°. If it’s obviously obtuse, then the angle measure should be between 90° and 180°. If you’re using the wrong set of numbers, your angle measure won’t make sense for your angle type.