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: Narrative
The purpose of this Number Talk is to elicit strategies and understandings students have for the structure of adding within 20. These understandings help students develop fluency and will be helpful later in this lesson when students use relationships between addends to make equivalent expressions to find sums.
Launch
Display one expression.
“Give me a signal when you have an answer and can explain how you got it.”
1 minute: quiet think time
Activity
Record answers and strategy.
Keep expressions and work displayed.
Repeat with each expression.
Student Facing
Find the value of each expression mentally.
\(5 + 8\)
\(6 + 7\)
\(8 + 7\)
\(6 + 9\)
Student Response
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Activity Synthesis
“Who can restate _____'s reasoning in a different way?”


aligns: no