standard_description: Use informal arguments to establish facts about the angle sum and exterior angle of triangles, about the angles created when parallel lines are cut by a transversal, and the angle-angle criterion for similarity of triangles. For example, arrange three copies of the same triangle so that the sum of the three angles appears to form a line, and give an argument in terms of transversals why this is so.
problem_activity: Narrative
The purpose of this Number Talk is to elicit strategies and understandings students have for making 10 when adding. These understandings help students develop fluency with operations within 20.
When students look for ways to rearrange and decompose numbers to make 10, they notice and make use of structure of expressions and whole numbers (MP7).
Launch
Display one expression.
“Give me a signal when you have an answer and can explain how you got it.”
1 minute: think time
Activity
Record answers and strategies.
Keep expressions and work displayed.
Repeat for each expression.
Student Facing
Find the value of each sum mentally.
\(4 + 8 + 2\)
\(4 + 5 + 3 + 2\)
\(9 + 3 + 1\)
\(9 + 5\)
Student Response
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Activity Synthesis
“How did making a ten help when finding the sums?”


aligns: no