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Illustrative Math
Grade 7
Let’s find ways to work with subtraction in expressions.
Illustrative Math Unit 7.6, Lesson 18 (printable worksheets)
The following diagram shows how to re-write subtraction as adding the opposite and then rearrange terms in an expression.
Find each sum or difference mentally.
-30 + -10
-10 + -30
-30 - 10
10 - -30
Lin and Kiran are trying to calculate 7 3/4 + 3 5/6 - 1 3/4. Here is their conversation:
Lin: “I plan to first add 7 3/4 and 3 5/6, so I will have to start by finding equivalent fractions with a common denominator.”
Kiran: “It would be a lot easier if we could start by working with the 1 3/4 and 7 3/4. Can we rewrite it like 7 3/4 + 1 3/4 - 3 5/6?”
Lin: “You can’t switch the order of numbers in a subtraction problem like you can with addition; 2 - 3 is not equal to 3 - 2.”
Kiran: “That’s true, but do you remember what we learned about rewriting subtraction expressions using addition? 2 - 3 is equal to 2 + (-3).”
Here is a calendar for April 2017. Let’s choose a date: the 10th. Look at the numbers above, below, and to either side of the 10th: 3, 17, 9, 11.
The average is 10.
The average is always the number in the middle.
The date above is 7 less, the date below is 7 more, the date to the left is 1 less, the date to the right is 1 more. The average is then the number in the middle.
The Open Up Resources math curriculum is free to download from the Open Up Resources website and is also available from Illustrative Mathematics.
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