Theorems On Triangles

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Karnataka state textbook: Class 8, Chapter 12-Theorems on triangles

Additional Information

This videos is related to types of triangle based on angles and side respectively.

Useful websites

  1. To know about classification of triangleclick here
  2. To know about properties of triangleclick here
  3. To learn about angle sum property of the triangleclick here
  4. Get your students to recognise, use and communicate formally with one another about triangles and their properties, using conventional terminology, i.e., 'mathematical talk'. Click here

Reference Books

  1. TamilNadu text book for theorems on triangle click here page no 94-102

Teaching Outlines

Concept #introduction to triangles

Learning objectives

  1. Define triangle and classify different types of triangle based on sides and angles.

Notes for teachers

First teacher can talk about or try to elicit basic points of geometry like lines, collinear and non collinear point, vertices,sides ect. this leads to the triangle definition.Then by using activity try to classify triangle based on side and angles and name them. teacher can also try to elicit live examples for triangle which are seen in daily life. Teacher can use the inducto deductive or CAM method.

Activity No 1 # classification of triangle based on sides

  • Estimated Time -30min
  • Materials/ Resources needed

cut outs of triangle , scale, pencil

  • Prerequisites/Instructions, if any

should know the meaning of triangle.and triangle is a 2 dimension figure

  • Multimedia resources

laptop, geogebra file

  • Website interactive/ links/ Geogebra Applets
  • Process (How to do the activity)
  1. Divide the class into groups and provide materials for each group
  2. Ask them to measure the length of the each side of the given triangles.
  3. Help them to identify the 3 types of triangles in that given triangles.
  4. ask them to try to name the identified 3 different types of triangle.
  • Developmental Questions (What discussion questions)
  1. what is the name of the given figure?
  2. how many types of triangle did you found?
  3. what name can you give for all this triangle?
  • Evaluation (Questions for assessment of the child)
  1. on what basis we classify the triangle?
  • Question Corner

Activity No 2 # Interior Angle Sum Property of a triangle

  • Estimated Time-45 minutes
  • Materials/ Resources needed
  • chart paper, pencil, scale, protractor
  • Prerequisites/Instructions, if any Students need to know:
    • a triangle is a three sided polygon.
    • types of triangles by sides and angles.
    • the characteristics of triangles by sides and angles.
    • facts of supplementary, adjacent and vertical angles.
    • write and solve simple linear equations.
  • Multimedia resources
  • Laptop
  • Website interactives/ links/ Geogebra Applets
  • Geogebra file on interior angle sum of a triangle, Lighthouse (ck-12 series)
  • Process (How to do the activity)
  • Students are taken through an inductive process of constructing a triangle using sliders for length of sides and an angle. The angle slider is moved to generate different types of triangles. The interior angle sum of different types of triangles are compared.
  • Developmental Questions (What discussion questions)
  • Evaluation (Questions for assessment of the child)
  • Question Corner

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