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Grade 4 · lesson 2 of 4

Classify a Triangle by Its Angles

Read the supplied angle measures of a valid triangle.

About 20 minutes, plus any extended practice

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Supplied materials

  • Paper and a pencil; drawing and spoken explanations are welcome. An adult may read directions and options aloud.
  • Supplied worked-case record: A triangle has angles 35°,55°,90°.
  • All quantities and shape descriptions are supplied in the worked example and questions; no purchase or personal information is needed.

Learn the idea

A right triangle contains one right angle. An obtuse triangle contains one angle greater than ninety degrees, and an acute triangle has all angles smaller than ninety. This classification uses angles; equal side lengths describe a different classification and cannot replace the angle evidence.

Read the supplied angle measures of a valid triangle. Look for ninety first, then a measure larger than ninety. If every angle is smaller, classify it as acute. Name the angle evidence supporting the decision.

Angle classification uses degree measures. One acute angle is not enough to prove the whole triangle is acute. List all three angles and circle any measure at least ninety degrees.

  1. List all three angles and circle any measure at least ninety degrees.
  2. Follow the worked example. Point to each number or shape feature and say what it tells you.
  3. Read the supplied angle measures of a valid triangle.
  4. Try the question and then the three practice questions. Check your answers and explain any change you make.
Worked example

See the method in action

A triangle has angles 35°,55°,90°. It is right because one angle is ninety. A triangle with 30°,40°,110° is obtuse, while 50°,60°,70° is acute. Each set totals 180°, but that total alone does not distinguish the three angle categories. Angle classification uses degree measures. One acute angle is not enough to prove the whole triangle is acute. List all three angles and circle any measure at least ninety degrees.

Your turn

Check your reasoning

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Helpful tips
  • Angle classification uses degree measures.
  • One acute angle is not enough to prove the whole triangle is acute.
A triangle has angles 20°, 70°, 90°. Classify it by angle.

Feedback appears here after you check. This is practice, not a graded school assessment.

See the answer and explanation

Right

A ninety-degree angle identifies a right triangle.

Take it further

Classify 15°,75°,90°. Expected: right triangle.

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More practice: try new cases

Choose an answer for each problem. Explain your method on paper, then use the feedback to decide what to revisit.

More practice: try new cases
1. A triangle has angles 30°, 50°, 100°. Classify it by angle.
Need a hint?
  • Angle classification uses degree measures.
  • One acute angle is not enough to prove the whole triangle is acute.
2. A triangle has angles 40°, 60°, 80°. Classify it by angle.
Need a hint?
  • Angle classification uses degree measures.
  • One acute angle is not enough to prove the whole triangle is acute.
3. A triangle has angles 45°, 45°, 90°. Classify it by angle.
Need a hint?
  • Angle classification uses degree measures.
  • One acute angle is not enough to prove the whole triangle is acute.

Your choices are temporary. Feedback appears after you check all the answers.

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Check the subject sources

These LifeStarter passages, scenarios and exercises are authored for this Academy with AI assistance and review. External resources keep their publishers’ terms. A grade label suggests a starting level; it does not establish alignment with your school’s complete requirements.