Reflection: Adjustments to Practice Solving Systems by Addition - Section 2: New Info

 

The systems in this lesson all have one variable in which the coefficient in one of the equations is the opposite of the coefficient of the other. In this case, "adding the equations" gives an equation in one variable and students solve the system pretty easily.

In systems that have a variable in which the coefficient in one equation is the same as the coefficient in the other equation, I prefer that students multiply one of the equations by -1, and then add.  My experience is that subtracting leads to too many sign errors, and this just interferes with the flow of the lesson. This especially holds true for those students who still struggle with operations involving signs. Some teachers like to teach both methods simultaneously. Yet, I've had better results not teaching subtraction and asking students to multiply by -1 and combine. This will be addressed in our next lesson.  

  Subtraction or multiplying by -1 to solve certain systems?
  Adjustments to Practice: Subtraction or multiplying by -1 to solve certain systems?
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Solving Systems by Addition

Unit 6: Systems of Linear Equations
Lesson 5 of 13

Objective: SWBAT solve systems in which adding on both sides, leads to an equation in one variable.

Big Idea: Knowing that the sum of the left and right sides of two different equations are equal, is key to solving certain systems, and later, many real world problems.

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Subject(s):
Math, Algebra, Systems of Equations and Inequalities, Solving Systems by Addition, linear combination, 8th grade math, linear equations, systems of equations, Math 8, Systems of, linear modeling
  65 minutes
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