Angles and Protractor Use

Angles measure rotation. Protractor is the control for measurement; compass+straightedge for construction. Indirect aim: preparation for trigonometry, geometric proof, angle bisector theorem.

• Framework: Montessori
• Unit: Mathematics
• Learning area: Geometry
• Level: Ages 8-10

Objectives

• Measure angles with a protractor in degrees
• Classify: acute, right, obtuse, straight, reflex
• Compute complementary and supplementary angles
• Bisect an angle with compass and straightedge

Materials

• Protractor
• Compass
• Straightedge
• Angle examples
• Recording paper

Instructions

1. Measure six angles in the classroom (door, book corner, open scissors). 2. Classify each. 3. Complementary: pair sums to 90°. Supplementary: pair sums to 180°. 4. Given 35°, find complement (55°) and supplement (145°). 5. Bisect 60° angle: compass+straightedge. 6. Verify: each half = 30°. 7. Draw angles of given size. 8. Record all work.

Educator tip

The compass bisection is one of Euclid's foundational constructions. Do it slowly; children love the precision.

Assessment

Observe measurement accuracy, classification, angle pair reasoning, and bisection.

More from Mathematics

• Ratio and Proportion
• Integer Operations — Signed Number Arithmetic
• Word Problems — Distance, Rate, Time
• Geometry Chart 1 — Basic Figures
• Congruence and Similarity
• Geometric Symmetry — Reflection, Rotation, Translation
• Measurement — Length in Metric
• Measurement — Weight and Mass