Lesson Plan: Intro to Triangles

Geometry is my favourite math unit – it relates to the real world, there are all kinds of manipulatives and students tend to enjoy it as well. This is how I like to introduce/review triangles with my grade 8 math students (this could suit any grade, the expectations would just differ):

Lesson One: Grade 8: Geometry Introduction/Review
Curriculum Connections:

  • Investigate and describe applications of geometric properties
  • investigate, describe and use the relationships between angles and intersecting lines
  • solve problems that involve lines and angles
Minds On (10 mins): Mini Number Talk

I love this website: http://wodb.ca/index.html

I project it on the board, and I choose the “shapes” category. Then I ask students to think independently about which shape doesn’t belong. After some time, I ask them to share with their group which one doesn’t belong, and that they need to use mathematical reasoning.

Then, as a group they need to agree on which one, and why – then each group will share with the class.

I use some of these questions to prompt discussion:

“Why did you choose that one?”
“Does another group have a different choice, or a different reason?”

“Do you agree?”

“Do you disagree? Why?”

“Which answer is the most correct? Is there one?”

Action (20 mins):

Students should work in pairs

Each pair receives some triangles (I’ve used pattern blocks, paper cut outs, tangrams etc.), a protractor and a ruler.

I ask each pair to categorize their shapes – based on whichever characteristics they’d like – but that they have to have mathematical justifications (not: blue and red etc.)

Once they’ve finished, I ask them to join up with another pair and to justify their sorting, after sharing, they must combine their triangles with that pair and come up with new sorting rules/characteristics.

I leave some discussion prompts on the board:

“Measure all the angles and sides on your triangles”

“Can you classify them in a different way?”

“What do you notice about the sum of all the angles of a triangle?”

Consolidate/Debrief (10 mins):

I like to bring the students back together to share what they’re thinking, things that they found and the rules they used to classify their shapes. I have groups share their classifications and groupings.


Because this is a formative and introductory activity, I usually circulate throughout to make sure students are on the right track, looking for previous knowledge and misunderstandings etc. I am also looking for their ability to use protractors, think about angles and find relationships between characteristics in triangles.

This is fun, hands-on and the students are doing most of the talking – which makes students and teachers happy!


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