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Math Help - Q in qeometric squence

  1. #1
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    Q in qeometric squence

    Hi
    If geometric sequence = a1 = 3 a7 = 2187 and r = 3 find S7
    in this Qustion which formula we use
    I use this : Sn = a1(1-r^n ) / 1- r
    the answer was = - 39
    please help me ..
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  2. #2
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     S_n = \frac{a(1-r^n)}{1- r}

     S_7 = \frac{3(1-3^7)}{1- 3}= 3279
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  3. #3
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    If r < 1 or r > 1

    also we use same formula

    <br />
S_n = \frac{a(1-r^n)}{1- r}<br />

    ???
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  4. #4
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    Yes, that's true. If r= 1, then the sum is na.
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