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n-th sum if pos. integers are grouped as 1, 2+3, 4+5+6, 7+8+9+10,...

Source: 1983 Swedish Mathematical Competition p1

March 28, 2021
Sumalgebra

Problem Statement

The positive integers are grouped as follows: 1,2+3,4+5+6,7+8+9+10,…1, 2+3, 4+5+6, 7+8+9+10,\dots. Find the value of the nn-th sum.
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