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Math Help - AP prob

  1. #1
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    AP prob

    the sum of n terms of an AP is 3n^2 +5. Find the number of terms which equals 159.
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  2. #2
    MHF Contributor
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    Quote Originally Posted by banku12 View Post
    the sum of n terms of an AP is 3n^2 +5. Find the number of terms which equals 159.
    hi

    T_n=a+(n-1)d=159 ---1

    We need to find a and d .

    a=S1=3(1)^2+5=8

    T_2=S_2-S_1

     <br />
=3(2)^2+5-8<br />

    =9

    so a=8 , and the common difference , d=1 . Now substitute into 1 to find n .
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