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Classifying Triangles by Angles: Acute, Right, and Obtuse

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Lesson Details

Grade Level
3-5
Duration (minutes)
2
Summary
Students learn to classify a triangle by finding its biggest angle and comparing it to 90°. An acute triangle has all three angles less than 90°, a right triangle has one angle of exactly 90°, and an obtuse triangle has one angle greater than 90°. A quick-check round gives practice with three more examples.

Classifying Triangles by Angles infographic: every triangle has 3 angles, and the biggest angle tells you its name. Three color-coded cards: Acute Triangle, all 3 angles less than 90° (70°, 60°, 50°); Right Triangle, 1 angle exactly 90°, marked with a small square (90°, 60°, 30°); Obtuse Triangle, 1 angle more than 90° (120°, 40°, 20°).

Student Worksheet: Classifying Triangles by Angles

Teacher Note: Students can watch the video lesson above and answer the questions. Click the “Teacher Answer Key” dropdown below when you are ready to review the solutions!

Key Concepts:

• Biggest Angle Rule: Every triangle has 3 angles. Find the biggest angle and compare it to 90° to name the triangle.

• Acute Triangle: All 3 angles are less than 90°.

• Right Triangle: 1 angle is exactly 90°. It is marked with a small square.

• Obtuse Triangle: 1 angle is more than 90°.

Instructions: Read each question carefully and write your answer in the space provided.

  1. Question 1: How many angles does every triangle have?
  2. Question 2: A triangle has angles of 70°, 60°, and 50°. What is its biggest angle, and what type of triangle is it?
  3. Question 3: How do we mark a right angle in a triangle?
  4. Question 4: A triangle has angles of 110°, 40°, and 30°. What type of triangle is it? Explain how you know.
  5. Question 5: A triangle has angles of 90°, 45°, and 45°. What type of triangle is it?
  6. Question 6: Challenge: A triangle has angles of 100°, 50°, and 30°. What type of triangle is it? Explain how you know.
Click to reveal Teacher Answer Key

1. 3 angles. @ 0:06

2. The biggest angle is 70°, which is less than 90°, so it is an acute triangle. @ 0:18

3. With a small square at the corner. @ 0:43

4. Obtuse, because the biggest angle (110°) is more than 90°. @ 1:24

5. Right, because it has a 90° angle. @ 1:17

6. Obtuse, because the biggest angle (100°) is more than 90°. (Application question — not shown in the video)

Disclaimer: This worksheet was generated with AI assistance based on original video content. It is intended for educational reference only. Teachers are encouraged to review and adjust questions to fit their curriculum standards before classroom use.

🎯 Extension Activity: Give students a stack of paper triangles in different shapes and the corner of an index card as a "90° tester." Have them test the biggest angle of each triangle and sort the triangles into three piles: acute, right, and obtuse.

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