Hands-on Critical Thinking: Equilateral Triangles

Equilateral TriangleAbout a month ago, I was working with my Geometry class on equilateral triangles. Using a ruler and a compass, I demonstrated how to construct an equilateral triangle. In fact, there are a number of websites that demonstrate this the same way that I did (see below).

After having the students practice drawing their own equilateral triangles, I introduced them to a two-part activity.

First, I gave them each a blank piece of paper and asked them to construct an equilateral triangle with each side equal to six inches. With a paper that is 8.5 in x 11 in, this was a fairly simple task for the students. Many of them were able to complete in a short amount of time. Those that struggled with it were allowed to ask their neighbor for support.

Second, I gave them a second blank piece of paper and asked them to construct an equilateral triangle with each side equal to twelve inches. At first, they looked at the paper and thought it was impossible. They struggled with it for quite a while on their own, sketching out different designs. Then, they asked if they could talk to one of their neighbors to brainstorm and explore different ideas. Soon, the students ended up in groups of three and four, thinking through all the mathematics they knew, trying to figure out how it could be done. Finally, one group asked if they could construct the triangle in parts, cut it out, and piece it back together. That question changed the whole atmosphere of the class. Quickly, every group saw the solution and were eager to share their approach with the class.

Steps for constructing an equilateral triangle with sides equal to twelve inches.

Step One – Measure six inches from the long side of the paper on both ends and mark the paper.


Step Two – Draw a line through both marks.


Step Three – From the bottom right corner of the inner rectangle, measure twelve inches so the twelve-inch mark of the ruler crosses the opposite side (thereby creating a diagonal).


Step Four – Measure the distance from the left side that the diagonal crosses the top side and mark that measurement below. Draw a line through both marks.


Step Five – Cut the two adjacent triangles and piece together to form an equilateral triangle.


The great part about this activity was that it led perfectly into our discussion of 30-60-90 triangles. Before exploring the properties of the 30-60-90 triangles, the students were already able to see their use in constructing (and forming) other geometric figures.


Best Practices: Diagonal of a Rectangular Prism

In class, I drew the following figure on the board:

Diagonal of a Cube

I asked the students if they could determine the length of the diagonal d given the information we’ve been recently covering. I’ve been talking to them about special right triangles (i.e. 45-45-90 and 30-60-90 triangles) at length to help them gain a deeper understanding of their properties as I prepare them for trigonometry next year. I even showed them how to use the distance formula to find the length of the hypotenuse. Yet, with all this knowledge and understanding, they weren’t even sure how to begin working on the problem.

I thought about this over the weekend and wondered if there were a different way, a more engaging way, that I could use to help them through this problem. So, I went to the store and bought several spools of string. I divided the class into groups of four and gave them each a spool of string. Then, I marked two opposite corners of the classroom and presented the challenge.

Group Challenge

Cut the string provided using only one cut so it may touch both corners when pulled tight. The groups could use any of the measuring devices provided (i.e. 12-inch ruler, yard stick, or measuring tape).

This required them to figure out how to use the measurements of the floor and the walls in determining the length of the string. By giving them the condition of only being able to cut the string once, they had to attend to precision (see CCSS.MATH.PRACTICE.MP6). Also, with a variety of measuring devices provided, they needed to determine which device would give them the most accurate measurement (see CCSS.MATH.PRACTICE.MP5).

Needless to say, this offered the students a different way of thinking about finding the diagonal of a cube. Some were able to derive the distance formula on their own as they worked through the calculations. Others admitted that they understand more now about square roots and working with triangles.

Alternative Assessments: Multiple-Choice Tests

Have you ever given a multiple-choice test and wondered whether restricting your students’ creativity to a small number of available options truly assessed their comprehension of a concept? I have, but I didn’t toss the multiple-choices out. Call me an optimist, but I always try to find the benefit of something.

Multiple ChoiceI asked myself, “What could possibly be the benefit of giving a multiple-choice test?” The last time I created a multiple-choice test, I found it far more involved than creating other types of assessments. Once I typed out the question, I knew the correct answer and struggled to write three more false answers for a total of four possible answers.  So, I wondered if I could harness some of that critical thinking that went into creating the multiple-choice test. What if I could take a different approach to this type of test that required a higher level of critical thinking? I thought for a while about it and decided to create a complete multiple-choice test, but without the questions.

That’s right! I gave my students a multiple-choice test with four answer choices (with the correct answer marked) for each question, but without the questions. Using the answer choices provided, they had to figure out what the question could have been. The first time that I tried it, the students seemed extremely engaged in the different way of thinking. What I liked about it was that it really got them to critically think about the material before writing down a question.

What alternative assessments have you created to inspire critical thinking?

Instructional Strategies: Project-Based Learning

PBL 2Project-based learning (PBL) is another student-centered model of learning that emphasizes 21st century learning skills (i.e. communication, collaboration, creativity, and critical thinking). According to the West Virginia Department of Education, “When engaged in standards-focused Project Based Learning (PBL), students are working in teams to experience and explore relevant, real-world problems, questions, issues and challenges; then creating presentations and products to share what they have learned.” There are many ways to implement PBL, but they all begin with problem situation. From that situation, the students analyze the situation provided and investigate possible solutions. They gather more information through research and assess their findings. Together, they develop a solution and prepare a presentation to share with the class.

PBL is not unique to mathematics education. In fact, PBL can be used in many content areas. Using PBL in mathematics education allows students to explore mathematical concepts in a more engaging way than working through a series of problems. Project-based learning provides a means of engaging students through social interaction, while providing a way to formatively assess the students’ understanding of the material. This is particularly important in mathematics, where students easily grasp the calculations, but struggle with the language used to explain the concepts and read the problems given to them. By increasing the amount of time students spend communicating and collaborating with their peers, they are simultaneously developing their critical thinking skills and their mathematical literacy.

Applying Bloom’s Revised Taxonomy to Mathematics

A few months ago, I designed a training on applying Bloom’s Taxonomy to create higher order thinking questions in mathematics.

I showed two figures: (1) A right triangle and (2) an equilateral triangle.


Right Triangle


Equilateral Triangle

The following were questions that I developed to help the instructors think through the various levels of Bloom’s Revised Taxonomy in regards to mathematics.

  • Knowledge – What can you tell me about the first triangle?
    • The students provide any information they know about the mathematical concept.
  • Comprehension – What makes the first triangle a right triangle?
    • The students use the information they already know about triangles to rightly identify a specific triangle.
  • Application – Based on what you know about right triangles, why is the second triangle not a right triangle?
    • The students apply the information they already know about triangle to differentiating one triangle from another based on their characteristics.
  • Analysis – How is the first triangle similar to a rectangle?
    • The students compare the characteristics of a right triangle with those of a rectangle.
  • Evaluation – How would you prove that all right triangles fit in a circle, with each vertex (or corner) of the triangle touching the circle?
    • The students extend their understanding of triangles by proving a well-known theorem of geometry (see Thale’s Theorem)
  • Synthesis – How could you use the right triangle to design our next engineering project?
    • The students integrate information they know and understand about the right triangle into designing a new project.

Any comments/suggestions?