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Think of addition and subtraction as
undoing each other.
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Examples:
 |
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8 +
7 =
15 |
put 8 and 7
together to get 15 |
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so: |
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15 -
8 =
7 |
"undo" 15 by
subtracting 8 |
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and |
|
|
15-
7 =
8 |
"undo" 15 by
subtracting 7 |
| |
|
| subtract 12 from 32 to get 20 |
32 -
12 =
20 |
 |
| |
so |
| give 12 back to 20 and get 32 |
20 +
12 = 32 |
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To solve equations, we use the fact that
subtraction an addition are inverses of each other all the time.
FOR
ADDITION
|
3 |
+ |
x |
= |
15 |
We are adding an unknown number to
3 to get 15 |
|
-3 |
|
|
|
-3 |
|
| |
|
x |
= |
12 |
"Undo" the addition by subtracting the
three from both sides. |
| |
|
|
|
|
|
 |
|
3 |
+ |
12 |
= |
15 |
The answer checks out! |
AND SUBTRACTION
| x |
- |
5 |
= |
23 |
Subtract 5 from an unknown number to get 23 |
| |
|
+5 |
|
+5 |
|
| |
|
x |
= |
28 |
"Undo" the subtraction by adding 5 to both sides. |
| |
|
|
|
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| 28 |
- |
5 |
= |
23 |
The answer checks out! |
BONUS:
To see a great interactive page that demonstrates adding and subtracting
on a number line, click here
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