# Engaging students: Solving linear systems of equations by either substitution or graphing

In my capstone class for future secondary math teachers, I ask my students to come up with ideas for engaging their students with different topics in the secondary mathematics curriculum. In other words, the point of the assignment was not to devise a full-blown lesson plan on this topic. Instead, I asked my students to think about three different ways of getting their students interested in the topic in the first place.

I plan to share some of the best of these ideas on this blog (after asking my students’ permission, of course).

This student submission comes from my former student Cameron Story. His topic, from Algebra II: solving linear systems of equations by either substitution or graphing. What interesting word problems using this topic can your students do now?

In algebra I, students are most likely to focus on a system of two equations with two unknown variables. Teachers incorporating two differently priced objects into a word problem works great as a real-world financial problem. However, these tend to be self-similar and are arguably uninspired. More importantly, students working to discover how to solve these systems are more challenged and engaged than those who are just handed the rulebook on systems of equations.

Suppose you place your students in the place of a farmer in ancient history. They have 25 different plots of land in their field, and each plot can either have a corn plant OR a wheat plant. However, suppose the farmer requires 4 times as many corn plants than wheat plants. Task your students to find out how many corn plants and how many wheat plants are in the 25-plot field, using any method they chose.

What is interesting is that there are multiple ways to solve this problem. Students could fill a 5×5 grid with labels C and W for corn and wheat. Then, making sure that they add 4 C’s for every W, they can simply count the squares in the grid to find the answer. Just from the information given to them, they could conclude that  and that . Students could then use substitution to arrive at the answer.

While many other methods arrive at the same solution, graphing these two equations on a W vs C graph reveals the answer to the student visually. After solving each equation for C in terms of W, the intersections of the two lines is the solution. Note that when showing this solution to your students, it is an opportune time to introduce what a system of equations with no solutions (parallel lines) or infinite solutions (two of the same line) look like. How does this topic extend what your students should have learned in previous courses?

Students are introduced to linear equations with the usual . In this equation, we have the one dependent variable y, whose value depends on the one independent variable x. When you first introduce a system of equations with two unknown variables, whose solution is some coordinate (x, y), the learning curve could be steep the students lack the conceptual understanding to connect linear equations to systems of linear equations.

You can then reveal to your students, or have them discover on their own, that you can take a system of two linear equations, write them in such a way that they represent two separate lines in point-slope form, and then find their intersection. If they intersect, then this is your (x, y) solution. Students should know that there is no coincidence here; just manipulation of something new into something more familiar. How can technology (graphing calculator websites or phone apps) be used to effectively engage students with this topic?

Say a student is solving a word problem that results in the following system of linear equations: $x-y=-1$ $x-4y=-2$

First the student is required to graph this system on an x vs y graph. On a typical handheld graphing calculator, this system cannot simply be punched into the calculator as is. The student might not know this yet, but their calculator could graph it after converting to point-slope form. However, the Geogebra (https://www.geogebra.org/graphing) website and mobile-app can take the equations as shown above as inputs directly without conversion. What I like most about having the students obtain the graph first is that it takes the system and transforms it into a 2-D graph of two intersecting lines. Students should know that each of these lines can be written as  . At this point with some further guidance, the relationship between the system of equations and the lines they represent in 2 dimensions should become apparent to the students through their own independent discovery.

References:

“Free Math Apps – Used by over 100 Million Students & Teachers Worldwide.” GeoGebra, http://www.geogebra.org/.

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