Evaluating Conditions for Congruency

Mathematical goals

This lesson unit is intended to help you assess how well students are able to:

  • Work with concepts of congruency and similarity, including identifying corresponding sides and corresponding angles within and between triangles.
  • Identify and understand the significance of a counter-example.
  • Prove and evaluate proofs in a geometric context.


In this lesson, students work on the concept of congruency whilst developing their understanding of proof in a geometric context.

  • Before the lesson, students complete a task designed to help you assess their current levels of understanding. You analyze their responses and write questions to help them improve their work.
  • The lesson begins with a whole-class discussion about establishing conditions for congruency from triangle properties. Students work alone to decide on the truth of a conjecture about congruency conditions for triangles. Then in pairs they share ideas and produce and justify a joint response. Working in the same pairs, they analyze sample responses produced by other students. In a whole-class discussion, students develop their understanding of proof in this context.
  • In a follow-up lesson students use what they have learned to improve their responses to the initial assessment task before attempting a second, similar task.

Materials required

  • Each student will need a copy of Finding Congruent Triangles and Finding Congruent Triangles (revisited), a rule, compasses, a protractor, the cut-up Card Set: Must the Two Triangles be Congruent? and Instructions: Must the Two Triangles be Congruent?, a mini-whiteboard, pen, and eraser and some paper to work on.
  • Each small group of students will need the Sample Student Proofs, some paper to work on and a glue stick.
  • A board compass, protractor and rule would be useful.

Time needed

20 minutes before the lesson, a 100-minute lesson (or two 55-minute lessons), and 20 minutes in a follow-up lesson. Timings are approximate and will depend on the needs of your students.