Let's Review
In a previous lesson, you learned that the unit of measure for an angle is a degree (°).You also learned that the degree of any angle can range from 0° to 360° and that the tool used to measure angles is called a protractor.
Remember that there are two scales on a protractor. On most protractors, the outside scale goes from 0° to 180° from left to right, or clockwise. The inside scale goes from 0° to 180° from right to left, or counterclockwise.
Angles are categorized as acute, right, obtuse, or straight based on their degree measure. Do you remember what those categories are? Review these types of angles by clicking on each tab below.
An acute angle is any angle that measures between 0° and 90°. Any angle that measures less than 90° is acute.
Keep in Mind
An angle that measure 0° is called a zero angle. It is not acute. An angle that measures 90° is a right angle. It is also not an acute angle.
A right angle measures exactly 90°.
Keep in Mind
An angle is only classified as a right angle when it is labeled 90° or it contains a small square in the inside corner of the angle as seen in this image. Never assume the classification of angle based on how the angle looks.
An obtuse angle is any angle that measures between 90° and 180°.
Keep in Mind
An obtuse angle cannot measure exactly 180°, as that angle measurement is classified as a straight angle.
A straight angle is an angle that measures exactly 180°, the measurement of a straight line. Straight angles occur wherever there is a straight line, including vertical and horizontal lines.
Keep in Mind
An angle is made up of two rays. Although this angle looks like one line, it is two rays that are pointing in opposite directions.
Let's Practice
Use the activity below to test your knowledge of all of the angle types. Practice classifying angles according to their measurement by matching each image on the left with the proper classification on the right.
