Angles: Types & Measure
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Learning Objectives
- 1Identify acute, right, obtuse, straight, and reflex angles by their degree range
- 2Measure an angle in degrees using a protractor
- 3Classify angles by comparing their measures to 90 and 180 degrees
- 4Estimate the measure of an angle before measuring to check reasonableness
Mini Lesson
An angle is the figure made by two rays that share the same endpoint, called the vertex. The size of an angle is measured in degrees and tells how far one ray has rotated away from the other. Angles show up in shapes, clocks, ramps, and almost every design you see.
The Five Angle Types
- Acute angle: measures less than 90 degrees — small and sharp.
- Right angle: measures exactly 90 degrees — looks like the corner of a square.
- Obtuse angle: measures more than 90 degrees but less than 180 degrees — wider than a right angle.
- Straight angle: measures exactly 180 degrees — the two rays form a straight line.
- Reflex angle: measures more than 180 degrees and less than 360 degrees — the larger way around.
Measuring with a Protractor
- Place the small hole or cross-hair at the center of the protractor on the vertex of the angle.
- Line up one ray of the angle with the 0-degree line on the protractor.
- Read the number where the other ray crosses the scale — use the scale that starts at 0 on your aligned ray.
- Write the measure with a degree symbol, like 47 degrees or 112 degrees.
Classifying What You See
- Compare the measure to 90 first: less than 90 is acute, equal to 90 is right.
- If the measure is greater than 90, compare it to 180: less than 180 is obtuse, equal to 180 is straight.
- Greater than 180 (but less than 360) is reflex — it is the angle measured the long way around the vertex.
- Always estimate before measuring: a sharp angle should not give a measure of 150 degrees.
Tip: estimate first. If the angle looks smaller than the corner of a book, your measure should be less than 90 degrees. If it looks bigger, expect more than 90 degrees.
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