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Last digit of 1+2+...+n

Source: Iberoamerican Olympiad 1992, Problem 1

May 14, 2007
number theory proposednumber theory

Problem Statement

For every positive integer nn we define ana_{n} as the last digit of the sum 1+2+⋯+n1+2+\cdots+n. Compute a1+a2+⋯+a1992a_{1}+a_{2}+\cdots+a_{1992}.
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