Tricky Triangles

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SWBAT identify, classify, and solve problems involving triangles.

Big Idea

Students apply what they know about attributes of triangles to solving problems.

Review and Refresh

15 minutes

Based on evidence from student work as well as what I've seen in students' reflections, this lesson was designed to provide students with additional opportunities to work with triangles. 

To start the lesson, I write TRIANGLES on the board and ask students to share what we know about the properties of triangles. I list their responses on the board to use as an anchor chart throughout the lesson.

• 3 sides, 3 angles

• All angles add to 180 degrees

• Triangles can be named for their sides (equilateral, isosceles, scalene)

• Triangles can be named for their angles (acute, right, obtuse)

This review and refresher allows students share what they know about the properties of triangles.  It also provides an opportunity for misconceptions to surface.  In the reflection for this section, some of these misconceptions are explained.

Next, I show students the triangles contain 180 degrees model from the Math is Fun website.  This page allows us to manipulate a triangle into infinite shapes and sizes.  As new triangles are created, the angle measures and total are shown, always adding to 180 degrees.

Before students are sent off to practice finding a missing angle, we work together to make sense of the triangle diagrams; discussing what the various symbols mean.  I also explain to students that if 2 sides are equal, than 2 angles will be too.  I do this quickly and add the measurements to the board to help students when they get to the 3rd and 4th examples.  At this point, they do not need to memorize this characteristic of triangles so I provide students with this information. 

Independent Practice

20 minutes

Students work in pairs to find the missing angle of a triangle.  They use diagrams to organize their thinking (part, part, part whole) and check their results using angle rulers. 

Challenges are provided for students to extend their thinking.  These "riddles" are from the Matholopis website, and are labeled as 5th grade hard.  

It is important to provide students with these challenges. Often times I think we shy away from things that seem like they are too hard for the students, claiming they won't be able to solve the problem.  

I like to provide students with a challenging task to push their thinking. 

Group Share

5 minutes

The schedule this week has continuously altered the time we have for math class.  The ticket out today, was a short group share that I left open ended.  

"Ask me a question about triangles".

We were able to answer just one group's question.

"When we were working with finding the missing angle, we got 82.1 when we did the math.  When we checked it with the angle ruler, we got 80 degrees.  Do we trust the math or the measurement more?"

This lead to a discussion about accuracy, scale, and precision. I told students to trust their math, because with the angle ruler, there is a chance of user error, since the tool is new to them.  They are more accurate with addition and subtraction.