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    Daily ChallengeHardSequences

    What is the sum of the first 20 terms of the arithmetic series: 2+5+8+11+…2 + 5 + 8 + 11 + \ldots?

    Qual é a soma dos primeiros 20 termos da série aritmética: 2+5+8+11+…2 + 5 + 8 + 11 + \ldots?

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    Solution

    a20=2+19×3=59a_{20} = 2+19 \times 3 = 59. S=20(2+59)2=20×612=610S = \frac{20(2+59)}{2} = \frac{20 \times 61}{2} = 610