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Casting Out Nines,弃九法;用于检查整数乘法、加法的结果是否错误。
表述
“Casting out nines” is an elementary check of a multiplication which makes use of the congruence 10^n \equiv 1 (\mod 9;mod 为取余数符号). Let decimal numbers be written a=a_n\cdots a_2a_1a_0, b=b_n\cdots b_2b_1b_0, and their product be c=c_n\cdots c_2c_1c_0. Let the sums of the digits of these numbers be a^\ast, b^\ast, and c^\ast. Then a\equiv a^\ast (\mod 9), b\equiv b^\ast (\mod 9), and c\equiv c^\ast (\mod 9). Furthermore ab\equiv a^\ast b^\ast (\mod 9), so c\equiv c^\ast (\mod 9). So if c and a^\ast b^\ast are incongruent (\mod 9), the multiplication has been done incorrectly.