Lesson Planning of Triangles

Lesson Planning of Triangles

Students` Learning Outcomes

  • Define a triangle.
  • Define triangles with respect to their sides [equilateral, isosceles and scalene triangles]
  • Define triangles with respect to their angles [acute – angled, obtuse-angled and right-angled]

Information for Teachers

  • Read the Grade-IV lesson-plans prepared for angles.
  • A triangle is one of the basic closed shapes of geometry. A triangle has three sides or edges and three angles.
  • Triangles can be classified in three ways based on sides and angles.
  • Sum of angles if a triangle is 1800 degrees
  • A triangle is a closed geometric figure formed by three lines, which intersect each other and which are not concurrent.
  • Classification of Triangles on the basis of Angles:

1.      Acute-Angled Triangle: A triangle each of whose angle is acute(Less than 900)

2.      Right-Angled Triangle: A Triangle one of whose angles is a right angle (equal to 900)

3.      Obtuse-Angled Triangle: A triangle with one of its angles as an obtuse angle (greater than 900 but less than 1800)


  • Classification of triangles on the basis of Sides:

a)      Scalene: A triangle, whose any two sides are equal.

b)      Isosceles Triangle: A triangle, any two of whose sides are equal.

c)      Equilateral Triangle: A triangle, all of whose sides are equal. 

Material / Resources

Writing board, chalk / marker, duster, shapes of all types of triangles traced on chart, four types of each type, sufficient number of bottles made with card board, pieces of straws of different lengths, pictures from library books or old new papers or from other textbooks (if possible), all types of card board


  • Distribute three straws of different lengths per pair of students and ask them to join them in a way that it makes a closed shape (a shape which is not open from any side). After a couple of minutes ask about the shape they made and let them express their work. Then introduce the term “Triangle”-Tri-means three, it is a shape with three angles. Tell them that we are going to talk about Triangles and its types.
  • With the help of your fingers make a triangle and ask them to do the same.(as shown in the figure)
  • Show them different architectural constructions which have triangles in its structure as shown in figures, in mosques also triangles shapes are used.
  • Then ask them to describe the shape, help them by saying it has three sides and three angles.
  • Ask them to look at the shapes they made with straws and see if it matches the shape made by fingers. Then introduce the definition as explained in key-points.


Activity 1

  • Let each students have a ruler and a worksheet which has pictures of three different triangles (one equilateral, one Isosceles, one scalene). Ask students that they can get pictures from books. Ask students to measure the sides of each triangle, labeling the picture with the appropriate lengths. Have them write down any observations they have about the differences between the three triangles.  
  • After students have had a few minutes to work on measuring, discuss their findings in the large group. Define scalene, isosceles and equilateral, pointing to which of the triangles fits each label.
  • Have students write definitions for these in their “geometry dictionaries”.
  • Distribute to each pair and ask them to measure its lengths and raise your hand when you have identified the type.
  • Collect the answers from each pair on board and then see if it matches your record when you prepared them.
  • Hence, reinforce the concept of classification with respect to lengths of sides.

Activity 2

  • Ask students the following question:
  • “Yesterday, we sorted triangles regarding their sides. Can you think of another way to sort triangles?” remind students that triangles have angles also.
  • Draw two different angles on the board. Ask students if they think that these angles are the same, citing their reasons why or why not.
  • Draw acute, obtuse and right angles on board and ask if they can recognize these anglers.
  • Make “L” shape with your fingers and ask do they remember the right angle. Then help them recall all the three angles.
  • Ask them to refer to the triangles from the last lesson. This time they have to measure its angles as well.
  • Let them measure all the three angles and come up with their relationship.
  • Then introduce the three types of triangles with respect to angles i.e. (acute-angled, obtuse-angled and right-angled triangle)
  • Then as a whole group discussion ask each pair to share their findings about angles and put them in a table on the board.


  • Write number of triangles of each group.
  • After collecting students’ feedback you know if it matches with the number you have while making those chart-paper triangles or not.
  • Give them few minutes to discuss all the types with reference to both angles and sides.
  • Invite students to share each of triangles with the whole group for the recap session.


Sum up / Conclusion

  • Ask students what they have learnt during the session about the triangles.
  • Ask them for the definitions. Later on repeat all the definitions for them.
  • Tell them that sum of angles in a triangle is 1800degrees.  


  • Work on word problem worksheets, straw constructions and homework.
  • Reflection Questions:

o   What did you learn about triangles today?

o   What did you like doing best?

o   What else did you learn about shapes today?

o   Can anyone think of different ways we can learn about triangles?

o   Can you name the objects in the classroom that has a triangular shape?

o   What other activities would you like to do with triangles and shapes?

o   Can we make different pictures and shapes with triangles?

o   What is the best thing you learned in class today?

o   How can we make learning about triangles more interesting?

o   Did we reach our goals and objectives today?

                                                                   More Questions:

  • Can you classify triangles according to the angles and sides?

1.1   Classification of triangles on the basis of the angles:

You can complete the following;

a)      Acute- angled triangles are triangles with……………………….

b)      Right-angled triangles have……………………………………………

c)       Obtuse-angled triangles have……………………………………….

1.2   Classification of triangles on the basis of the sides:

Are you able to complete the following?

a)      An isosceles triangle has………………………………………….

b)      An isosceles triangle has………………………………………..

c)       A scalene triangles’ …………………………………………… 

  • Recognize the triangles given above.
  • Involve the students in solving the problems given at the end of the lesson of textbook.

Follow up

  • Ask the students to repeat the concepts that they have learnt today. Write the suggestions of students on the board.  Define angles with collective approach and students should write it in their “Geometry Dictionary”
  • Tick the right option;


1 thought on “Lesson Planning of Triangles”

  1. I am curious to find out what blog system you have been working with?I’m experiencing some minor security problems with my latest blog and I’d like to find something more safe.Do you have any recommendations?

Leave a Comment