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# Complement

Complement is a group of proteins found in blood serum which act in concert with antibodies to achieve the destruction of non-self particles such as foreign blood cells or bacteria.

Complement is also a mathematical operation.

The complement of a number is a tool used to subtract numbers when only addition is allowed. I will explain complements for our familiar decimal number system.

To find the complement of a number, you need to know how many digits you want your complement to have. Then, make the substitutions in the table below and then add 1 to the result.

 The digit 0 1 2 3 4 5 6 7 8 9 changes to 9 8 7 6 5 4 3 2 1 0

Example: Find the complement of 7520. We want 5 digits.

• Using the table, 07520 becomes 92479.
• Add 1 to get 92480.

To subtract two numbers using complements, you take the first number and add it to the complement of the second number, then erase the "1" at the beginning of the sum.

Example: From 375 subtract 82.

• Since the longest number we have is three digits, we will use a three-digit complement.
• The complement of 082 is: (917 plus 1) which is 918.
• 375 plus 918 is 1293. Erase the "1" at the beginning to get the final answer.

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