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last digit of 1 + 2 + 3 + ...+ n,

Source: Polish MO Finals 1952 p5

August 28, 2024
number theorylast digits

Problem Statement

Prove that none of the digits 22, 44, 77, 99 can be the last digit of a number 1+2+3+…+n, 1 + 2 + 3 + \ldots + n, where nn is a natural number.
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