(2) Decimal complement subtraction. This process involves the addition of a number and the
complement of the second number being subtracted. This achieves subtraction by addition.
(a) Subtraction by the 10's complement. A number (subtrahend) may be subtracted from
another number (minuend) by adding the 10's complement of the subtrahend to the minuend.
The difference between 42 and 39 is not 103. With this method, an extra-order 1 is generated. This 1
signifies the answer is a positive number and is dropped (103 - 100 = +3). The answer then is +3 which is
the correct difference between 42 and 39.
Here are some other examples of this method.
(b) These two examples involved positive answer. Let's now consider the situation in which
the subtrahend is larger than the minuend. The difference in such cases will be a negative quantity. This
demonstrates a negative difference.
Note that in this example, an extra-order 1 was not generated; this means the answer is a negative
number. To find the answer, the 10's complement of 978 must be found and written with a negative sign.
IT0339
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