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Homework Help: Local Extrema

  1. Oct 28, 2013 #1


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    Gold Member

    1. The problem statement, all variables and given/known data

    http://i.minus.com/jZdpOtdOiChOn.jpg [Broken]

    2. Relevant equations

    Local extrema can be determined using the first derivative test.

    3. The attempt at a solution

    I ran the first derivative test to find the critical points, which were 0 and plus/minus 0.5. I plugged in the values into the original equation. x = 0 makes the function go to infinity, so x = 0 can be ruled out as any sort of local extrema. x = 0.5 makes the function = 2sqrt(e), while x = -0.5 makes the function = -2sqrt(e). Naively, I chose D, which pegs x = 0.5 as the local maximum, which makes sense, doesn't it?

    Unfortunately the formal definition of a local maxima is that the sign of the first derivative changes from positive to negative, and in the case of x = 0.5, the opposite happens; the sign actually flips from negative to positive around it, making it a local minima.

    I'm assuming this is the correct explanation.

    Who else would have fallen for this? Let's be honest :P.
    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. Oct 28, 2013 #2


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    Homework Helper

    I knew people who were fooled by this sort of problem. It is possible that a local maximum be "lower" than a local minimum. Look at the graphs of secant or cosecant, for example.
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