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Math Help - alternatives to second derivative test

  1. #1
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    alternatives to second derivative test

    Hi all, I have been working on the old "what is the largest cylinder by volume that can inscribed in a sphere of radius r" question. The solution is easy enough, but proving that the point at which dv/dr = 0 (where v is the volume of the cylinder and r = the radius of the circular base of the cylinder) is a maximum by using the second derivative test seems ridiculously laborious when it is obviously a maximum. Logically we can say that this point can't be a minimum because the minimum would have to be r = 0, v =0. But how I can prove that it's not a horizontal POI without reverting to the 2nd derivative test.

    is there a way to prove this logically withouth having to find the second derivative?
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  2. #2
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    Re: alternatives to second derivative test

    well, there is one way that doesn't require a second derivative, but depending on the function you're "testing" it may not be any less work.

    if f'(a) = 0, compare f(a-ε), f(a), and f(a+ε), where ε is a very small number.
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