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Find the number of terms and the sum of the geometric progression 8, 4, 2, ..., 1/512.

Mathematics

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Answer

16

Step-by-step solution

4 steps

This is a geometric progression.

  1. Identify the first term, common ratio, and last term.

    The first term is a=8a = 8.
    The common ratio is r=48=12r = \frac{4}{8} = \frac{1}{2}.
    The last term is l=1512l = \frac{1}{512}.

  2. Find the number of terms (nn).

    The formula for the nn-th term of a geometric progression is l=a⋅rn−1l = a \cdot r^{n-1}.
    Substitute the known values:

    1512=8⋅(12)n−1\frac{1}{512} = 8 \cdot \left(\frac{1}{2}\right)^{n-1}

    Divide both sides by 8:

    1512×8=(12)n−1\frac{1}{512 \times 8} = \left(\frac{1}{2}\right)^{n-1} 14096=(12)n−1\frac{1}{4096} = \left(\frac{1}{2}\right)^{n-1}

    Since 212=40962^{12} = 4096, we can write:

    (12)12=(12)n−1\left(\frac{1}{2}\right)^{12} = \left(\frac{1}{2}\right)^{n-1}

    Equating the exponents:

    12=n−112 = n-1 n=13n = 13

    There are 13 terms in the progression.

  3. Calculate the sum of the geometric progression (SnS_n).

    The formula for the sum of the first nn terms of a geometric progression is Sn=a(1−rn)1−rS_n = \frac{a(1 - r^n)}{1 - r} (since ∣r∣<1|r| < 1).
    Substitute a=8a=8, r=12r=\frac{1}{2}, and n=13n=13:

    S13=8(1−(12)13)1−12S_{13} = \frac{8 \left(1 - \left(\frac{1}{2}\right)^{13}\right)}{1 - \frac{1}{2}}

    Calculate (12)13\left(\frac{1}{2}\right)^{13}:

    (12)13=1213=18192\left(\frac{1}{2}\right)^{13} = \frac{1}{2^{13}} = \frac{1}{8192}

    Substitute this back into the sum formula:

    S13=8(1−18192)12S_{13} = \frac{8 \left(1 - \frac{1}{8192}\right)}{\frac{1}{2}} S13=16(1−18192)S_{13} = 16 \left(1 - \frac{1}{8192}\right) S13=16−168192S_{13} = 16 - \frac{16}{8192} S13=16−1512S_{13} = 16 - \frac{1}{512} S13=16−0.001953125S_{13} = 16 - 0.001953125 S13=15.998046875S_{13} = 15.998046875
  4. Round the sum to the nearest whole number.

    Rounding 15.998046875 to the nearest whole number gives 16.

    The sum of the progression, rounded to the nearest whole number, is 16\boxed{16}.

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