A 13-year-old student found an easy way to multiply numbers with repeating digits by nine. The theorem developed by Enzo de Oliveira Pimenta states that when multiplying a number formed by a digit that repeats x times—for example, 3,333,333, in which the digit 3 repeats seven times—by nine, you simply need to multiply the repeating digit by nine, which will give a two-digit number y (27, in this case). The result of the multiplication will be the first digit of y followed by x-1 nines and then by the second digit of y: 29,999,997, in our example. “The theorem is linked to the principle of finite induction and could help in teaching this mathematical concept,” says Enzo’s father, Marcos Pimenta, a mathematician at São Paulo State University (UNESP). The principle of finite induction is based on the assumption that a statement that is correct for one natural number is true for the next number. Enzo developed his theorem while studying a Kumon course. His father encouraged him to prove the formula and helped him write an article about it, published in the journal Professor de Matemática Online (Impartial, October 17).
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