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As in addition, no two quantities can be subtracted unless they are measured or reported in the same units. Thus, you cannot subtract 7 hundreds from 9 tens and expect to get 2 of anything. A quantity can only be subtracted from another quantity if we can first find a common unit between the two.
The operation of subtraction can be thought of as:
• | a separator, when the result, starting point, or change is unknown (also known as "take-away") |
• | a comparison, when the result, starting point, or change is unknown |
• | a missing addend problem, where one of the parts is unknown Note 4 |
This table gives an example of each kind of subtraction problem:
The missing addend problems are written as addition problems, but the procedure to solve these problems requires the use of some subtraction strategy. The separating concept always requires some sort of separating action, whereas the comparison concept is static.
When negative numbers are introduced, we can more clearly understand the concept of subtraction as the addition of the inverse. Thus, we can write 13 - 6 as the equivalent of 13 + (-6 ), because -6 is the additive inverse of 6; i.e., 6 + (-6) = 0. Similarly, we can represent 13 - (-6) as 13 + 6, since 6 is the additive inverse of -6. So again, subtracting a number (13 - (-6)) is the same as adding its inverse (13 + 6).
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