Those are all the complements you ever have to remember in adding the longest column of figures. There are only five of them: five pairs, you will note, that add up to ten in the table of possible combinations.
Before learning how to add with complements, make doubly sure that you have the idea by looking at the following digits and giving their complements. Try to "read" the complement of each. 7 6 9 5 8
The way you add with complements takes a bit of getting used to. But it is one of the most fascinating and fruitful approaches known to short-cut arithmetic. You "add" two digits that total more than ten by subtracting the complement of the larger digit from the smaller digit and recording a ten.
In order to add 6 + 7, you subtract the complement of 7(3) from 6, and record a ten. 6 - 3 gives 3. The recorded ten makes it 13.
Or to add 8 + 4, you subtract the complement of 8 (2) from 4 and record a ten. 4 - 2 gives 2. The recorded ten makes it 12. It is useful to subtract the complement of the larger digit rather than the complement of the smaller. In this way you cut in half the number of complements you have to remember at this stage - though the other half of the complements are really only the same pairs of digits that add up to ten turned around. Just as 2 is the complement of 8, so is 8 the complement of 2.
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