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1 September, 07:19

What is the sum if the first 22 terms of the following series? 86 + 79 + 72 + 65 + 58 + ...

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  1. 1 September, 08:23
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    Seems to be an arythmetic sequence

    Sn=[n (a1+an) ]/2

    where

    Sn means sum of all terms up the nth term

    n=number of terms

    a1=first term

    an=nth term

    so from 86 to the 22th term is from a1 to a22

    find teh sequence

    miknus 7 each time

    an=a1+d (n-1)

    an=87-7 (n-1)

    find 22n term

    a22=87-7 (22-1)

    a22=87-7 (21)

    a22=87-147

    a22=-60

    S22=[22 (87-60) ]/2

    S22=[22 (27) ]/2

    S22=594/2

    S22=297

    the sum is 297
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