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19 December, 01:11

I got this problem from an old algebra book and I seem to have got a mental block with Part 2:

Part 1. Find the nth term for the sequence 1, 3/2, 5/4, 7/8 ... ?

Part 2. If the sum of n terms is denoted by s, write down the series obtained by subtracting 1/2s from s, and find its value in terms of n. Hence find s.

I obtained nth term = (2n - 1) / 2^ (n-1) for Part 1.

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  1. 19 December, 01:18
    0
    Step-by-step explanation:

    S = 1 + 3/2 + 5/4 + 7/8 + ...

    ½S = 1/2 + 3/4 + 5/8 + 7/16 + ...

    Subtracting:

    S - ½S = 1 + 3/2 - 1/2 + 5/4 - 3/4 + 7/8 - 5/8 + ...

    Simplifying:

    S - ½S = 1 + 1 + 1/2 + 1/4 + ...

    Writing in summation form:

    S - ½S = 1 + ∑ ½ⁿ⁻¹

    ½S = 1 + ∑ ½ⁿ⁻¹

    S = 2 + 2 ∑ ½ⁿ⁻¹

    Using formula for sum of a geometric series:

    S = 2 + 2 (1 / (1 - ½))

    S = 2 + 2 (2)

    S = 6
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