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Homework Help: Elipse, tangents, and limits.

  1. Oct 21, 2011 #1
    1. The problem statement, all variables and given/known data, is best expressed in the photograph, as this is a fairly detailed problem:
    [PLAIN]http://img833.imageshack.us/img833/3972/pleasehelp.jpg [Broken]
    2. all equations included in the image
    3. the attempt at the solution is in the image as well

    When I take the limit of that last equation, I get a nonsensical answer--infinity. I am well sure that its not infinity. If anything, it must be negative infinity.

    What am I doing wrong? Is it my simplification? Is there a strategic error?

    Please help!!

    Found the same question here with some searching:

    the answer is not given, but that's a different approach, the person used the y=mx+b way.
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Oct 21, 2011 #2


    Staff: Mentor

    I didn't check your work, but there seems to be something wrong with your last line:
    h(c) = (a2 - c2)/c + c
    This simplifies to
    h(c) = (a2 - c2 + c2)/c = a2/c.

    From your drawing, c is positive, and a2 is positive, so h(c) should be positive, but the point on the y axis is clearly negative. I suspect that you were not as careful as you need to be with the choice of pos/neg square roots in your preceding work.
  4. Oct 23, 2011 #3
    It's actually (b^2-a^2)/b for the answer... dont know why, but the y-mx+b yielded the correct answer... less room for mistakes there!

    thanks a lot though Mark44
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