Inverse Operations & Complements
Addition and subtraction (also multiplication and division) are the inverse of each other. It means that they undo each other. So, just as 9 + 5 = 14, so also 14 – 5 = 9.
We can use this fact to find missing numbers, e.g. to calculate 9 + ? = 14 what calculation are we actually doing? What about 7 + ? = 21? What about 35 + ? = 81?
Let’s try some questions to practice this skill, and in general to practice using complements to answer questions like 100 – 17.
Adding & Subtracting Numbers
Do we have a good written method that will let us add or subtract any numbers? Let’s try it for a few and see if we all have a good effective method (some students may have seen different methods for this).