standard_description: Prove that, given a system of two equations in two variables, replacing one equation by the sum of that equation and a multiple of the other produces a system with the same solutions.
problem_activity: Activity
The purpose of this activity is to give students an opportunity to practice checking whether a particular value is a solution to an equation, and to recall properties of operations and equality that preserve the solution set of an equation.
This will be useful when students consider when and why equations have the same solution in the associated Algebra 1 lesson.
Monitor for students who:
check the value of
\(x\)
in only one expression and reason about the second expression
identify properties by name such as commutative, distributive, and associative
identify properties informally such as “changing the order doesn’t matter” or “you can add first and then multiply or multiply first and then add” or “you can multiply three or more terms in any order” or “you can add the same thing to each side”
manipulate the expressions
graph one or both expressions
Making graphing technology available gives students an opportunity to choose appropriate tools strategically.
Student Facing
For each pair of equations, decide whether the given value of
\(x\)
is a solution to one or both equations:
Is
\(x = 2\)
a solution to:
\(x(2 + 3) = 10\)
\(2x + 3x = 10\)
Is
\(x = 3\)
a solution to:
\(x - 4 = 1\)
\(4 - x = 1\)
Is
\(x = \text{-}2\)
a solution to:
\(7x = \text{-}14\)
\(x \boldcdot 14 = \text{-}28\)
Is
\(x = \text{-}1\)
a solution to:
\(x + 3 = 2\)
\(3 + x = 2\)
Is
\(x = \text{-5}\)
a solution to:
\(3 - x = 8\)
\(5 - x = 10\)
Is
\(x = (8 + 1)+3\)
a solution to:
\(\frac{12}{2} = \frac12(x)\)
\(18 = 2x\)
Is
\(x = 2\)
a solution to:
\(\frac{12}{x} = 6\)
\(6x = 12\)
Is
\(x = \frac{10}{3}\)
a solution to:
\(\text{-}1 + 3x = 9\)
\(9 = 3x - 1\)
Is
\(x = \frac12\)
a solution to:
\(5(x + 1) = \frac{15}{2}\)
\(5x + 1 = \frac{15}{2}\)
Student Response
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Activity Synthesis
Ask students to talk to their neighbor about some things they can look for, to see if two equations are likely to have the same solution or different solutions. If needed, draw students’ attention to which equations had the same solution and which had different solutions, perhaps by displaying them for all to see, grouped by whether the equations were equivalent or not.
Call on students to share their thinking and make an informal list displayed for all to see. It’s fine if students do not name all possible properties, or only describe them informally. They will have more opportunities to encounter them formally in their Algebra 1 course.
Remind students that
equivalent
equations have the same solution set. Use the language of
equivalent
interchangeably with “have the same solution,” and monitor for students to pick up on this vocabulary.


aligns: no