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Sequences and series question

  1. Jul 31, 2011 #1
    1. The problem statement, all variables and given/known data
    I wrote a test and the question was something like this

    2, 4, 6..... 108

    It said... " Find tn"

    Does this just mean find any term number that isnt given? I just plugged in t5 for the arithamtic logic and solved.. dunno if it was right, does anyone know?


    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Jul 31, 2011 #2
    Bump for someone giving a one word answer like "yes" or "no" based on what i presented
     
  4. Jul 31, 2011 #3
    It's hard to understand from what you wrote, but I think the answer they were going for was "2n".
     
  5. Jul 31, 2011 #4
    How is that hard to understand....


    >given series

    > 2, 4, 6..... 108

    > Solve tn

    Thats all it said. So i just solved any random term. In the book it usually gave a term like solve for t4 . But this time it didnt.

    Does it just mean solve for an unknown.
     
  6. Jul 31, 2011 #5
    Then it means solve for the nth term. The answer is 2n.
     
  7. Jul 31, 2011 #6

    PeterO

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    Homework Helper

    The book said find tn, why did you chose t5?

    I think you probably have a couple of formulae for tn, in terms of the first term [sometimes called "t1", sometimes called "a"] along with either the common difference [d] or the common ratio [r] depending whether it is an arithmetic or geometric sequence.

    You were required to recognise which sequence this one is, identify the first term and common difference or ratio then substitute in the general formula to get a specific expression for tn.
    Once you have that, t4 is what you get when you replace n with 4.

    I suspect you have worked with the answer, without realising it was the answer.
     
  8. Jul 31, 2011 #7
    Yes, ive worked with the answer. I jus realsied its the same thing as when they give you two terms.

    ie) t1= 2
    t2 = 4 ... Then ask you to find tn.

    Finding tn, meaning getting the equation for both points then using elimination and solving for d and a , etc.

    6 marks lost for no reason woo^^
     
  9. Jul 31, 2011 #8

    PeterO

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    Homework Helper

    Might I suggest that when preparing for an exercise on Sequences & Series, you prepare your self a list of things to find BEFORE reading the question. Reading the question will only distract you!

    That list would include:

    First term, Common Difference or Common Ratio, Formula/expression for the nth term, the value of a specific term, formula/expression for the Sum of a group of terms, etc...

    You then read the question to see which one or more of those things they asked you for this time.
     
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