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2014-11-09T20:38:25+02:00
2021+2×(1+2+3+.....+2020)
2021+2×2020×(2020+1)÷2
2021+4040×2021÷2
2021+2020×2021
2021×(2020+1)
2021×2021=
=2021^2

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