Objective: Model n – n and n – (n – 1) pictorially and as subtraction sentences.
The discussion in Lesson 34 around what happens each time students take away 0 or 1 with numbers within 10 leads them to an understanding that this same reality remains true with all numbers. Similarly, students explore what happens in both n – n and n – (n – 1) situations. They notice, “When I take 5 away from 5, I’m left with zero every time.” and “5 – 4 is 1, just like 6 – 5 is 1.” Students generalize their understanding: “Any number minus a number that’s one less will leave us with just 1.”
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