Subtracting a negative integer: three models to justify the rule
I show three models or ways to think about subtracting a negative integer: counters, number line jumps, and thinking about the distance between the numbers. Each way verifies (justifies) the SHORTCUT that subtracting a negative number is the same as ADDING a positive number: it is as if the two negatives − − turn into a positive + .
(Of course, this shortcut is just a specific instance of the general principle that subtraction really is adding the opposite. I don't mention this in the lesson though, as the lesson is meant as introductory, and for 6th grade.)
After seeing those models/ways, we then proceed to solve some practice problems (additions and subtractions) using the shortcut.
See also
Add and subtract integers on a number line — a video lesson
Addition of integers — a video lesson
Math Mammoth Integers — a self-teaching worktext with explanations & exercises for all operations with integers
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