Solving the Pythagorean Theorem Algebraically Completed

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SWBAT understand the structure of the Pythagorean Theorem algebraically in order to solve for an unknown side length as an equation.

Big Idea

Make sense of structure and move into solving the Pythagorean Theorem Algebraically


10 minutes

As a review of solving equations, put the following two equations on the board and ask students to solve each equation.  Solutions should be rounded to the nearest hundredth. 


Solve for x:   45 = 7(x – 3)                           (5x)2 + 30 = 155

 While students are working to solve each equation, move about the room formatively assessing students, providing feedback to struggling students, and choosing correct work for students to share on the board to each question.  If possible, choose students to present who wrote or thought about the equations in different ways in order to view a variety of thinking.  Have each student present their own work and explain their thinking.  

Another great method of assessing every student, is to have students use dry erase markers and marker boards to show their work.  You could show the expressions one at a time and put one minute or thirty seconds even on a clock and allow only that much time to simplify then hold up their boards when the time was up.  This way, you can quickly see everyone’s thinking without needing to walk about the room so much.  The marker boards are faster and you can keep a scratch sheet of paper to list the names of all students who seem to struggle through the bellringer as your formative assessment. 

 Again, because the activity today is focused around connecting the Pythagorean Theorem to solving equations, this review of solving equations that are both linear and quadratic are important as the lesson opener.  You might want to just mention that not all quadratic equations are so simple to solve.  Sometimes, a quadratic equation takes on  the form 3x2 – 7x + 42 = 18.  What is it about this quadratic equation that makes it more difficult to solve for the unknown value?

Beginning the Activity

20 minutes

Clarifying and Sharing Learning Goals

Always begin by clarifying for the students what it is they will be learning from the activity today.  

The learning question for today is: “How do I think about using the Pythagorean Theorem algebraically to solve for missing side lengths?”  I want to push as many students as possible into thinking about the Pythagorean Theorem algebraically and connecting the solving for a missing side length to solving equations, even liner equations as in previous unit.  However, I always have some students who are just more successful with visual strategies.  I do not force my students to write and solve the Pythagorean Theorem algebraically.  Many visual learners like to draw the squares on the diagram and still consider area, moving area of squares, and then finding the side lengths of a square to find the missing sides of the right triangle.  I allow these students to continue using this strategy, which is why I have allowed so much exterior space around each diagram.  It is my hope that most students will begin to transition into solving algebraically as an equation, but I do still allow choice in solving.  Later when trying to develop the distance formula from the Pythagorean Theorem, a sound algebraic understanding of the Pythagorean Theorem is very useful.  In high school geometry, students will be required to use the distance formula algebraically because it is also directly connected to the equation of a circle as I mentioned in the lesson rational video.


Starting the Activity

Once your expectations and learning goals are clear, you may want to spend about three minutes showing answers to the homework problems in question six.  Take questions from the students about problems that were difficult and put a conclusion on finding the missing hypotenuse lengths.  If I know I want to review these answers because students were not asked to work in Edmodo the night before, then I give candy to students who voluntarily put answers with work shown on the board as part of the bellringer process. This makes looking at answers to the homework much faster. 


Next, allow students about five minutes to work within their cooperative groups to answer questions seven through ten.  Prepare students that the diagram in these questions has changed and they must now decide if the change in the diagram also changes the student work.  Students must decide which student has worked the problem correctly and then describe in full sentences the exact mistakes of the other two students.  It is important for students to write in full sentences and coherent thoughts what the specific mistakes are for the two incorrect students.  Again, a goal of this activity is to bring to light all the most common mistakes students make, so that your current students will not repeat these incorrect actions.

As students are working within groups to complete questions seven through ten, move about the room providing feedback that moves learning forward.  It is common for some groups to say, “I don’t know who is right.  They all look the same.”  I usually ask these groups what is the correct answer to the question?  Have you worked out the problem your way yet to consider who might be correct?”  Usually these groups have not begun to solve the problem for themselves and this suggestion is enough to get them started in a productive direction. 

Also, as you move about the room from group to group, select students or groups for presentation during the mini-wrap-up session.  Choose student work that is correct and possibly worked or explained in different ways.

After allowing about five minutes or a little longer if needed for groups to working through questions seven through ten bring the whole class back together and hold a mini wrap up for students to share what they have learned.  This mini wrap up should be student lead and student focused so students feel that they own their own learning and can discuss and defend it to others.  Click below to watch a short video about how I incorporate student ownership of learning in my classroom. 

I call this time of students presenting their work a “mini wrap-up” because I do not spend long periods of time closing a lesson at the end of the class period.  We use small lesson closers after a small chunk of material has been completed.  If you are unsure of what a mini-wrap up looks like, click below to watch a short video on how I use the mini wrap-up strategy.

Throughout the mini-wrap up, either the student groups or you should be scripting important ideas from the presentation onto the whiteboard for preservation.  Many students zone out or cannot follow verbal explanations because there are visual learners.  Scripting important ideas on the board for all students to use a guide map throughout the class period is very important.  For further explanation of scripting, click on the follow link to watch a short video about how and why I believe scripting is vital to student success.

Group Work

20 minutes

Working in Cooperative Groups to Make Connections Continued

Once you conclude a mini wrap up of student work, allow students about five minutes to complete questions 11, 12, and 13  Prepare students for these questions by saying, “The following two questions are scaffolded questions designed to help you ease into setting up and solving the Pythagorean Theorem algebraically.  The blank lines are missing values in each step of solving the Pythagorean Theorem as an equation.  Question 13 is designed for you to think about how the first example in question one and second example in question seven are both alike and different in how you think about them and solve them.”   Students should work together to solve these equations as you also move about the room providing feedback.  

After allowing students time to finish questions 11, 12, and 13 pull three student groups to the board to present their work as a mini wrap up for the class.  Make sure either the students or you script the entire solution onto the white board for notes and reference as a map throughout class.  Discuss question 13 well so that students understand you cannot always use a and b for the sides you know and simply solve for c in every triangle.  This misunderstanding is very common and students do not understand why sometime you subtract and sometimes you add before taking the square root.  Make sure students understand the structure algebraically of each equation.


Where to End the Lesson

I try to make it to question 14 which contains practice problems solving for the hypotenuse and sometimes solving for missing legs, with about five to ten minutes left in class so that students can begin to work together to solve for the missing hypotenuse.  Homework is to finish all of the practice problems in question 14.  I again allowed extra white space around each diagram for students who plan to draw the three squares on each side in order to solve.  I encourage the algebraic equation method but do not force it.  For my classes with technology at home, I often require students to take pictures of their homework answers to question six and post these images through their Edmodo account to the rest of the class.  I then require students to logo into Edmodo and also post at least two comments to another student that night in  If you are unsure of how to use or even what it is, click below to watch a short video of how I have used to link my students this school year.


Student Work and Standards Addressed

Here are a few images of sample work produced by my students throughout this school year.  The samples are of answers to the first five questions only.

 Standards Applied in this Lesson

The math standard addressed by this lesson is 8.G.B.7 Apply the Pythagorean Theorem to determine unknown side lengths in right triangles in real-world and mathematical problems in two and three dimensions.

MP3 Construct viable arguments and critique the reasoning of others 

Asking students to think algebraically about setting up and solving the Pythagorean Theorem to the nearest hundredth brings in math practice standards 6 and 8. 

MP6 Attend to precision. (Click here to watch a short video on MP6)

MP8 Regularity in Math