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Labelling and classifying triangles

This page was translated from German with AI. It has not been checked by a native speaker. Original page in German

You learn how to label the corners, sides and angles of a triangle, and you learn the types of triangles.
📄 Open worksheet (PDF, German)

The video

In the video a triangle is labelled step by step. Then you see five types of triangles.

Subtitles are available in ten languages. Choose them at the bottom right of the video.

Contents of the video

  1. 0:00 Introduction
  2. 0:18 The corners
  3. 0:28 The sides
  4. 0:40 The angles
  5. 0:52 Classifying by angles
  6. 1:21 Classifying by sides
  7. 1:54 Careful, mistake!
  8. 2:09 Try it yourself

The most important points

Labelling a triangle

Corners (die Ecken): \(A\), \(B\), \(C\) – start at the bottom left, go anticlockwise. Sides (die Seiten): \(a\) is opposite \(A\), \(b\) opposite \(B\), \(c\) opposite \(C\). Angles (die Winkel): \(\alpha\) (alpha) at \(A\), \(\beta\) (beta) at \(B\), \(\gamma\) (gamma) at \(C\).

Corners, sides and angles are labelled.
Corners, sides and angles are labelled.

By angles

acute (spitzwinklig): all angles smaller than \(90^\circ\) · right-angled (rechtwinklig): one angle exactly \(90^\circ\) · obtuse (stumpfwinklig): one angle bigger than \(90^\circ\).

Acute, right-angled, obtuse.
Acute, right-angled, obtuse.

By sides

isosceles (gleichschenklig): two sides the same length, the base angles (Basiswinkel) are equal · equilateral (gleichseitig): all sides the same length, every angle \(60^\circ\).

Isosceles and equilateral. The marks show sides of the same length.
Isosceles and equilateral. The marks show sides of the same length.

The worksheet

Here you can see the worksheet for the video (in German). First it explains everything again, then there are practice tasks at three levels. You can also open it as a PDF and print it.

📄 Open worksheet (PDF, German)

The worksheet – Page 1
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The worksheet – Page 2
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The worksheet – Page 3
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The worksheet – Page 4
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Show the worksheet solutions
The worksheet – Page 5
Page 5/5

Try it yourself

Task

Which type of triangle? a) angles \(40^\circ\), \(50^\circ\), \(90^\circ\) b) sides 6 cm, 6 cm, 6 cm c) angles \(25^\circ\), \(120^\circ\), \(35^\circ\)

Show solution

a) right-angled b) equilateral c) obtuse

Keep learning with a language model

You are my maths teacher. I am learning German as a second language. Our topic: labelling triangles (Ecken A, B, C, Seiten a, b, c, Winkel Alpha, Beta, Gamma) and types of triangles (spitzwinklig, rechtwinklig, stumpfwinklig, gleichschenklig, gleichseitig). Give me five short tasks, one after the other. Always wait for my answer. Then tell me if it is right and explain briefly why. Use simple German sentences.

How this was made: The idea, structure and examples of the series are mine. I made them with the help of AI: a language model (LLM) built the slides of the video following my instructions. The voice was generated with Qwen3-TTS, modelled on my own voice.

The subtitles, the translations, the worksheet and this page were also made with the help of AI. The translations have not been checked by native speakers.

The videos, worksheets, texts and prompts in this series are licensed under CC BY 4.0: you may use, change and share them if you name the source.