Casting out nines is a sanity test to ensure that hand computations of sums, differences, products, and quotients of integers are correct. By looking at the digital roots of the inputs and outputs, the casting-out-nines method can help one check arithmetic calculations. The method is so simple that most schoolchildren can apply it without understa...
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http://en.wikipedia.org/wiki/Casting_out_nines
A method for checking arithmetic that uses the idea of the digital root of a number. Let the digital root of a number n be r(n); for example, r(7586) = 8. For any two numbers a and b: r(a + b) = r(r(a) + r(b)) and r(a × b) = r(r(a) × r(b)). These rules allow checks on addition and multipli...
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http://www.daviddarling.info/encyclopedia/C/casting_out_nines.html
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