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math

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the ninth term of an arithmetic sequence is 23 and the 33rd term of the sequence is -241 , calculate the sume of the first 15 terms of the sequence , using the formula

s=n(2a+(n-1)d)/2

  • math -

    a+8d = 23
    a+32d = -241
    subtract them ...
    24d = -264
    d = -11
    back into the first ....
    a - 88 = 23
    a = 111

    now that we know a = 111, d = -11 and n = 15
    just plug those values into your formula

  • math -

    Sn=510

  • math -

    S15 =510

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