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Math Help - my teacher marked this wrong on the test

  1. #1
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    my teacher marked this wrong on the test

    should i argue with her, or is this just wrong.

    i got 0 credit for it, instructions of the test were:
    "fore each of the follwoing, determine wheter the infinite series converges or diverges. if it is an alternating series, determine whether it coverges conditionally or absolutely.

    she wrote on the test "geometric series converges" which i see works but how about this:




    is that mathmatically wrong somewhere?
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  2. #2
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    Quote Originally Posted by Legendsn3verdie View Post
    should i argue with her, or is this just wrong.

    i got 0 credit for it, instructions of the test were:
    "fore each of the follwoing, determine wheter the infinite series converges or diverges. if it is an alternating series, determine whether it coverges conditionally or absolutely.

    she wrote on the test "geometric series converges" which i see works but how about this:




    is that mathmatically wrong somewhere?
    Yes it's wrong. Your teacher is quite correct to give you zero credit for this solution. It shows a fundamental lack of understanding of series algebra.

    The result of the nth root test is 2/5, NOT 5*(2/5).

    Once you take the 5 out of the summation, your a_n is \left(\frac{2}{5}\right)^n.

    Once the 5 is taken out, all it does is multiply with the value that the series converges to.
    Last edited by mr fantastic; December 10th 2008 at 04:07 AM. Reason: Corrected latex error
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  3. #3
    Grand Panjandrum
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    Quote Originally Posted by Legendsn3verdie View Post
    should i argue with her, or is this just wrong.

    i got 0 credit for it, instructions of the test were:
    "fore each of the follwoing, determine wheter the infinite series converges or diverges. if it is an alternating series, determine whether it coverges conditionally or absolutely.

    she wrote on the test "geometric series converges" which i see works but how about this:




    is that mathmatically wrong somewhere?
    The n-th root of the n-th term is \sqrt[n]{5 (2/5)^n} not 5 \sqrt[n]{(2/5)^n}

    Alternativly you should be applying the n-th root test to the series \sum_{n=1}^{\infty}(2/5)^n


    CB
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  4. #4
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    hm yah ok i guess i was completly wrong thanks for helpin me understand my error.
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