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Calculators Sequences and Sums with TI-89

  1. Dec 1, 2007 #1

    I'm trying to figure out my TI-89. So I want to estimate the 40th partial sum of this series:

    Sum(40) of (-1^(k+1))/k^4, starting at k=1. My major problem is that I want to use 'k's or 'n's, not 'x's. Is there a difference? I haven't asked my Calc teacher about this yet, but I know that 'k's and 'n's deal with integers while 'x's encompass the real numbers.

    When I do the sum with 'x', I get a a number -1.08232 but the book shows .094703.

    I hate the way that all these manuals show n and k, and presume you can input those variables -- but I haven't seen an 'n' or 'k' variable, and when I use the capital letters it doesn't work.
    Last edited: Dec 1, 2007
  2. jcsd
  3. Dec 2, 2007 #2
    The 'n' and 'k' are typically called 'indices.' In mathematics, the convention is that 'n' and 'k' are often used to represent whole numbers, and 'x' often represents real numbers. Generally, to a calculator, 'n' and 'x' are just variables. It may be that 'x' is indeed dedicated to real values; perhaps there is a corresponding "delta" x which regulates the increments of the value 'x'. For example, when graphing, the "delta" x determines how far apart each point is plotted before "connecting the dots."

    Also, check the syntax for the sum() command to see if you're using it properly. And be careful (esp. with TIs) when using -1^(k+1); to be safe, I'd write this as (-1)^(k+1).
  4. Dec 8, 2007 #3
    So if I use x when all these instructions tell me to use 'n' or 'k', it shouldn't matter?
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