Systems of Linear Equations

05 Oct

We are working on Systems of Linear Equations in our Algebra 1 class.

The first day, our lesson goals were the following:

Level 4: I can create a system of linear equations to solve a problem.

Level 3: I can solve a system of linear equations, determining whether the system has one solution, no solution, or many solutions.

Level 2: I can graph a linear equation y=mx+b in the x-y coordinate plane.

Level 1: I can determine whether an ordered pair is a solution to a linear equation.

We used the Mathematics Assessment Project formative assessment lesson Solving Linear Equations in Two Variables to introduce systems.

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Then we asked students to draw a sketch of a system of equations with one solution, a sketch of a system of equations with no solution, and a sketch of a system of equations with many solutions.

We used the Math Nspired activity How Many Solutions to a System to let students explore necessary and sufficient conditions for the equations in a system with one, no, or many solution(s). We used a question from the activity so that students could solidify their thinking about the relationship between the equations for different solutions.

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We sent the questions as Quick Polls so that we could formatively assess their thinking and correct misconceptions.

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We like using this type of Quick Poll (y=, Include a Graph Preview) while students are learning. The poll graphs what students type so that they can check their results and modify the equation when they do not get the expected result.

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(Note: on the second day, we asked again for students to create a second equation to make a system with one, no, and many solution(s), but the given equation is in standard form.)


On Day 2 of Systems, the learning goals are slightly different.

Level 4: I can solve a system of linear equations in more than one way to verify my solution.

Level 3: I can create a system of linear equations to solve a problem.

Level 2: I can solve a system of linear equations, determining whether the system has one solution, no solution, or many solutions.

Level 1: I can determine whether an ordered pair is a solution to a linear equation.

We started with a task from Dan Meyer’s Makeover series, Internet Plans. Part of this makeover was a complete overhaul of the context. Students write down a number between 1 and 25. They view this flyer (from Frank Nochese).


And then they decide which gym membership to choose, using the number they wrote down as the number of months they plan to work out.


When I saw the C’s and B’s on the number line on the blog post, I actually thought that the image was beautiful – a clear indication that one plan is better than the other two for a while, and then another plan is better than the other two. This image absolutely leads to the question of which one becomes better when – and thinking about why plan A is never the better deal.

Our students started calculating. Several asked what $1 down meant before they could get a calculation. Then they shared their work on our class number line.

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We were dumbfounded at the results. Really. It was hard to know where to start. We were prepared to help student develop the question with correct calculations. We hadn’t thought about what we would do when students couldn’t figure out which plan was the better deal for a specific, given number of months.

What would you do next if this is how your students answered?


We started with the lone C at 4 months, hoping someone would claim it and share his calculations. He did: I divided $199 by 12 months and then multiplied by 4 to get the cost for 4 months.


Another student said he couldn’t do that because you had to pay $199 for 12 months no matter how many months you actually used it.


Another student said that A was the better deal because they were planning to get every other month free by working out more than 24 days each month. We discussed how realistic it is to work out at a gym 24 days a month.


We did eventually look at a graphical representation of each plan. And talked about what assumptions had been made using these graphs.

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Can you tell which graph is which plan?

Can you write the equations of the lines?

In the first class we never made it to actually calculating the point(s) of intersection (class was shorter because of a pep rally), but other classes did calculate the point(s) of intersection.

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It’s not a waste of time to think about Plan C costing $199 for 12 months.

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But then we really should have created a piecewise function to represent the cost for 13-24 months as well.

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The number lines weren’t much better in subsequent classes.


So maybe the students got to practice model with mathematics, just a little, even though we have a long way to go before students will be able to say, “I can create a system of linear equations to solve a problem”.

The teachers were definitely reminded of how much work we have to do this year, as the journey continues …


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4 responses to “Systems of Linear Equations

  1. howardat58

    October 5, 2014 at 8:34 pm

    Poor Dan and Emma ! Did you give them this to figure out?

    I have a feeling that the computer is creating problems with the notation. You have 3 equations, so there are three lines, each with a y=mx+b, but the computer seems to want to use functional notation. Where are the y’s ? And I wonder whether the computer can show the ax+by=c form.

    The intro about creating an equation which together with y=1/2x+3 has many solutions suggests that it will be different from the given one. Not so, it’s just a copy. This is a more sensible question when using the ax+by=c form.

    Maybe I’m just being fussy !

  2. travis

    October 6, 2014 at 12:17 pm

    good stuff.
    fyi, you can edit the function names. This takes time, but might be worth it.


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