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Angles - Acute, Obtuse, Straight, and Right

Resource for Grades 9-12

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Angles - Acute, Obtuse, Straight, and Right

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Geometry
(Video)

Angles are classified as either acute, obtuse, straight or right. The measure of the degree of the angle determines the classification type.

These resources are part of KET's Measurement and Geometry collection.

This video segment originally appeared in KET’s GED Connection series.

Background Essay

An angle is formed when two portions of a line share a common endpoint, or vertex. Angles are measured in degrees. The symbol for degrees is ^o. A right angle measures exactly 90^o (90 degrees). The corner of a sheet of paper is a right angle. Right angles are formed when two lines are perpendicular. In a diagram, a right angle is marked with a square symbol. Right angles are very useful for solving geometric problems.

An angle smaller than 90^o is an acute angle. An angle larger than 90^o is an obtuse angle. A straight angle, which looks no different from a line, measures exactly 180^o.

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Discussion Questions

What are some ways you can help yourself remember the definitions for each of the types of angles?
For what professions would you need to measure angles?

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Teaching Tips

Find examples of each type of angle in the room. Have learners use small adhesive-backed notes to identify and temporarily label the angles.

Have learners draw a clock face and identify times when the hands on the clock form acute, obtuse, straight, and right angles.

Ask learners why they think right angles are used in construction more than any other angle.