Subject : 



Definition and Types of Triangles


Term :




Class :


Previous knowledge:

The learners have previous knowledge of       Geometric Construction (Circles)          that was taught as a topic during the last lesson


Behavioural objectives:

By the end of the lesson, the pupils should be able to

  • define triangle
  • mention types of triangles
  • construct any types of triangles


Instructional Materials:

  • Wall charts
  • Pictures
  • Related Online Video
  • Flash Cards

Methods of Teaching:

  • Class Discussion
  • Group Discussion
  • Asking Questions
  • Explanation
  • Role Modelling
  • Role Delegation


Reference Materials:

  • Scheme of Work
  • Online Information
  • Textbooks
  • Workbooks
  • 9 Year Basic Education Curriculum
  • Workbooks




A triangle may be defined as a plane figure contained by three straight lines, known as the sides of the triangles. One of the sides, usually the one most nearly horizontal, is identified as the base.

A triangle, as its name implies, has three angles. The two angles at the ends of the base are called base angles. The third angle, which is opposite the base, is called the vertical angle is formed is termed the vertex of the triangle.

The sum of the three angles of any triangle is 180 degree

You can check this statement by drawing a triangle in any manner you like. Measure the three angles formed using a protractor. Sum the angles and confirm that they add up to 1800.

The altitude or height of a triangle is the perpendicular distance from the vertex to the base.

Triangles are classified according to the magnitude of their angles or according to the length of their side.

  1. Types of Triangles according to Angles

An acute angle triangle is one which has each of its three angles less than right angle.

right angled triangle is one which has one of its as a right angle. The side of the triangle opposite the right angle is called the hypothenuse.

An obtuse angled triangle has one has one of its angles greater than a right angle.

  1. Types of Triangles according to sides

An equilateral triangle is one which has its sides equal.

(Note: The three angles are also equal, each being 600.)

The sides are also equal i.e., AB = BC =AC. An isosceles triangle has two of its sides equal.

(Note: The two angles of the feet of the equal sides are also equal.)

scalene triangle is one in which no side is equal to another.


  1. Define Triangle?
  2. The sum of the three angles of any triangle is___








Presentation: The topic is presented step by step


Step 1:

The class teacher revises the previous topics


Step 2.

He introduces the new topic


Step 3:

The class teacher allows the pupils to give their own examples and he corrects them when the needs arise


  1. Define Triangle?
  2. The sum of the three angles of any triangle is___


The class teacher wraps up or concludes the lesson by giving out a short note to summarize the topic that he or she has just taught.

The class teacher also goes round to make sure that the notes are well copied or well written by the pupils.

He or she makes the necessary corrections when and where the needs arise.









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