standard_description: Use similar triangles to explain why the slope m is the same between any two distinct points on a non-vertical line in the coordinate plane; derive the equation y = mx for a line through the origin and the equation y = mx + b for a line intercepting the vertical axis at b.
problem_activity: Activity
The goal of this activity is for students to build fluency solving equations with variables on each side. Students describe each step in their solution process to a partner and justify how each of their changes maintains the equality of the two expressions (MP3).
Look for groups solving problems in different, but efficient, ways. For example, one group may distribute the
\(\frac12\)
on the left side in problem 2 while another may multiply each side of the equation by 2 in order to re-write the equation with less factors on each side.
Teacher Notes for IM 6–8 Math Accelerated
This activity is optional. Depending on the time available, adjust the total number of cards used by groups.
Launch
Arrange students in groups of 2. Instruct the class that they will receive 4 cards with problems on them and that their goal is to create a solution to the problems.
For the first two cards they draw, students will alternate solving by stating to their partner the step they plan to do to each side of the equation and why before writing down the step and passing the card. For the final two problem cards, each partner picks one and writes out its solution individually before trading to check each others’ work.
To help students understand how they are expected to solve the first two problems, demonstrate the trading process with a student volunteer and a sample equation. Emphasize that the “why” justification should include how their step maintains the equality of the equation. Use MLR 3 (Clarify, Critique, Correct) by reminding students to push each other to explain how their step guarantees that the equation is still balanced as they are working. For example, a student might say they are combining two terms on one side of the equation, which maintains the equality as the value of that side does not change, only the appearance.
Distribute 4 slips from the blackline master to each group. Give time for groups to complete the problems, leaving at least 5 minutes for a whole-class discussion. If any groups finish early, make sure they have checked their solutions and then challenge them to try and find a new solution to one of the problems that uses less steps than their first solution. Conclude with a whole-class discussion.
If time is a concern, give each group 2 cards rather than all 4 and have them only doing the trading steps portion of the activity, but make sure that all 4 cards are distributed throughout the class. Give 6–7 minutes for groups to complete their problems. Make sure each problem is discussed in a final whole-group discussion. Alternatively, extend the activity by selecting more problems for students to solve with their partners.
Student Facing
Your teacher will give you 4 cards, each with an equation.
With your partner, select a card and choose who will take the first turn.
During your turn, decide what the next move to solve the equation should be, explain your choice to your partner, and then write it down once you both agree. Switch roles for the next move. This continues until the equation is solved.
Choose a second equation to solve in the same way, trading the card back and forth after each move.
For the last two equations, choose one each to solve and then trade with your partner when you finish to check one another’s work.
Student Response
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Activity Synthesis
Depending on your observations as students worked, you may wish to begin the discussion with a few common errors and ask students to explain why they are errors. For example, write out a solution to problem 2 where the second line has
\(3.5x-6\)
instead of
\(3.5x-3\)
and then ask students to find the error.
The goal of this discussion is for the class to see different, successful ways of solving the same equation. Record and display the student thinking that emerges during the discussion to help the class follow what is being said. To highlight some of the differences in solution paths, ask:
“Did your partner ever make a move different than the one you expected them to? Describe it.”
“For problem 4, could you start by halving the value of each side? Why might you want to do this?”
“What’s an arithmetic error you made but then caught when you checked your work?”
Representation: Internalize Comprehension.
Chunk this task into more manageable parts to differentiate the degree of difficulty or complexity by beginning with fewer cards. For example, give students cards a subset of the cards to start with and introduce the remaining cards once students have completed the initial set, or create an additional set of cards with equations that are more accessible.
Supports accessibility for: Conceptual processing
Speaking: MLR8 Discussion Supports.
Use this routine to support students in producing statements about common errors in problem solving. Use the example offered, (the second line has
\(3.5x−6\)
instead of
\(3.5x−3\)
for problem 2), and provide sentence frames to support the discussion. For example, “The error this student made was… and I believe this happened because….” and “A different solution path could be….” Restate or revoice student language to demonstrate use of correct mathematical language to describe each move (e.g., “distribute the
\(\frac12\)
,” “combine like terms,” etc.), and include mathematical reasoning (e.g., “...because this maintains the equality”). Clarify explanations that detail differences in problem-solving strategies rather than errors, to help students see differences in solution paths. This will help students describe differences in solution paths and justify each step.
Design Principle(s): Cultivate conversation; Maximize meta-awareness


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