Complementary angles

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Complementary angles are two angles with a sum of 90°. A common case is when they form a right angle.


Introduce children to concept of complementary angles

Estimated Time

30 minutes

Prerequisites/Instructions, prior preparations, if any

Prior knowledge of point, lines, angles

Materials/ Resources needed

  • Digital : Computer, geogebra application, projector.
  • Non digital : Worksheet and pencil.
  • Geogebra files : Complementary angles.ggb

Download this geogebra file from this link.

Process (How to do the activity)

  • Prior hands on activity (optional – children can use news paper strips to make angles and place them together and notice if the resulting angle formed is a right angle).
  • Students should be able to identify the types of angles in the file
  • Let them attempt to join the two angles -They may try by coinciding different points or lines.
  • Ask about the angle formed by joining the two angles?
  • Challenge them to find another way to make a right angle using the two existing angles.
  • Interchanging the position of the two angles – ask what they notice.
  • If one angle is changed will the two together again form a complementary angle.
  • Children can be asked to make note of the angle measures for different angles work sheet
  • Observations made with angles seen so far.
  • Ask if the angles have to be adjacent to be complementary pairs
  • Record the values of the two angles and their sum in the worksheet
Sl No. Value of slider α Angle ABC Angle DEF Sum

Angle ABC + Angle DEF

Is angle ABC complement of angle DEF
  • Evaluation at the end of the activity
  1. What are complementary pair of angles?
  2. Is it required for the pair of angles to be adjacent to be complementary?
  3. What types of angles form complementary pair?