Use Protractors for Angles
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What you will learn
- 1Identify acute, right, obtuse, and straight angles
- 2Measure angles using a protractor
- 3Classify angles as less than, equal to, or greater than 90°
Mini Lesson
An angle is formed where two rays meet at a point called the vertex. We measure angles in degrees (°) using a tool called a protractor. The size of the angle tells us how far one ray has turned from the other.
Types of Angles
- Right angle: exactly 90° — shown with a small square symbol at the vertex.
- Acute angle: less than 90° — a sharp, narrow angle.
- Obtuse angle: between 90° and 180° — a wide, blunt angle.
- Straight angle: exactly 180° — two rays pointing in opposite directions, forming a straight line.
Using a Protractor
- Place the center hole of the protractor exactly on the vertex of the angle.
- Align the base line (0°) of the protractor along one ray of the angle.
- Read the scale where the second ray crosses the numbered arc.
- Read the scale (inner or outer) that starts at 0° on the ray you lined up.
Words to know
angle
The figure formed when two rays meet at a common point.
e.g. The hands of a clock at 3:00 form a right angle.
vertex
The point where the two rays of an angle meet.
e.g. Place the center of the protractor on the vertex.
degree (°)
The unit used to measure the size of an angle.
e.g. A right angle measures 90 degrees.
protractor
A semicircular tool used to measure angles in degrees.
e.g. Use a protractor to find that the angle is 60°.
acute angle
An angle that measures less than 90°.
e.g. A 45° angle is acute.
obtuse angle
An angle that measures more than 90° but less than 180°.
e.g. A 120° angle is obtuse.
Tip: always check whether your angle looks acute (sharp) or obtuse (wide) before reading the protractor. If your reading does not match the look of the angle, you have read the wrong scale.
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