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Math Help - How does one find the sum of series by multiplying through

  1. #1
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    How does one find the sum of series by multiplying through

    For example cos(x)+cos(2x)+...+cos(n-1)x
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  2. #2
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    Re: How does one find the sum of series by multiplying through

    Use the identity cos(a)=(exp(ia)+exp(-ia))/2 which transforms the sum into geometric series.
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  3. #3
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    Re: How does one find the sum of series by multiplying through

    The hint is 2sin(x/2). Do I begin by just multiplying every term by that.. I don't know how to start.
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