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5 February, 11:46

Find the sum of the first 20 terms of the arithmetic sequence - 16, - 20, - 24?

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  1. 5 February, 13:29
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    An = a1 + (n - 1) * d

    a20 = - 16 + (20 - 1) * - 4

    a20 = - 16 + (19 * - 4)

    a20 = - 16 - 76

    a20 = - 92

    sn = n (a1 + a20) / 2

    s20 = 20 (-16 - 92) / 2

    s20 = 20 (-108) / 2

    s20 = - 2160/2

    s20 = - 1080 < = = the sum of the first 20 terms
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