Overview
This lesson covers how to measure different types of angles using a protractor, including acute, right, obtuse, and reflex angles.
Parts of a Protractor
- A protractor has a baseline and a center point at the middle.
- It usually has two measuring scales, one starting from the left and one from the right.
- Both scales measure from 0° to 180°.
Measuring Angles with a Protractor
- Align the baseline of the protractor with one arm of the angle.
- Place the center point exactly on the vertex (corner) of the angle.
- Read the measurement from zero on the correct side, moving towards the other arm.
Examples of Measuring Angles
- Angle ABC: Measured using the left scale; angle is 40°, which is an acute angle (less than 90°).
- Angle PQR: Measured using the right scale; angle is 65°, also an acute angle.
- Angle DEF: Measured using the left scale; angle is 90°, a right angle.
- Angle XYZ: Measured using the right scale; angle is 123°, which is an obtuse angle (between 90° and 180°).
Measuring Reflex Angles
- Reflex angles are larger than 180° but less than 360°.
- To measure a reflex angle, subtract the measured (smaller) angle from 360°.
- Angle MNL: The smaller angle is 130°, so reflex angle = 360° – 130° = 230°.
- Angle FGH: The smaller angle is 40°, so reflex angle = 360° – 40° = 320°.
Key Terms & Definitions
- Protractor — A tool used for measuring angles in degrees.
- Vertex — The common endpoint where two arms of an angle meet.
- Acute Angle — An angle less than 90°.
- Right Angle — An angle equal to 90°.
- Obtuse Angle — An angle between 90° and 180°.
- Reflex Angle — An angle greater than 180° but less than 360°.
- Revolution — A complete turn, equal to 360°.
Action Items / Next Steps
- Practice measuring different types of angles using a protractor.
- Review definitions of angle types for the upcoming test.