math

1. Addition and Subtraction
+ -
Finding the Inverse

2. Question?
What does
inverse mean?

3. Definition
Inverse means to "undo"
OR something similar

4. For example…
You could say that messy is the inverse
of a clean (they are opposites and
“undo” each other).

5. In math, we say that…
• Addition is the inverse of subtraction
• Subtraction is the inverse of addition
Because these two operations “undo”
each other

6. Let’s Count on to 20
1 2 3 4
5 6 7 8
9 10 11 12
13 14 15 16
17 18 19 20

7. Let’s Add
4 1 5
+ =

8. So…
4 + 1 = 5
What about 1 + 4? Does that equal 5 too?
Let’s see…
1 + 4 = 5
YES!

9. Let’s Count back from 20
1 2 3 4
5 6 7 8
9 10 11 12
13 14 15 16
17 18 19 20

10. Let’s Subtract
- =
6 4 2

11. So...
6 – 4 = 2
GREAT!
Watch what happens when I switch
the “-” and the “=” signs and make
the “-” a “+”
6 = 4 + 2

12. = +
6 4 2
6 = 4 + 2
you can also write it like this…
4 + 2 = 6 OR 2 + 4 = 6

13. Let’s try that again!
5 – 3 = 2
--Switch the signs around--
5 = 3 + 2
You could also write it like this…
3 + 2 = 5
OR
2 + 3 = 5
(+)

14. Let’s try that one more time!
This time we are going to start with addition
5 + 4 = 9
4 + 5 = 9
--Switch the signs--
(remember, when doing subtraction you ALWAYS start with the bigger number)
9 – 4 = 5
OR
9 – 5 = 4

15. Here are some more examples of the
inverse at work…
9 + 1 = 10 10 – 1 = 9
1 + 9 = 10 10 – 9 = 1
2 + 2 = 4 4 – 2 = 2
6 + 4 = 10 10 – 4 = 6
4 + 6 = 10 10 – 6 = 4

16. So…what is the inverse?
The inverse is what undoes the first
equation
Addition can undo subtraction and
subtraction can undo addition

17. Let’s keep practicing!