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Math Help - local maximum problem

  1. #1
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    local maximum problem

    Find the decreasing interval, increasing intervals, and local maximum of the following function:

    f(x) = 6x^3-18x^2-144x-7

    The derivative is :
    f'(x) = 18x^2-36x-144

    The decreasing interval is (-2, 4)
    The increasing intervals are (-inf, -2) and (4, inf)

    I thought the local max was found by putting in the critical values ( in this case -2 and 4) into the original function. f(-2) = 161 and f(4)= -427.
    So the local maximum = 161, right? When I submit this answer I'm told that it is incorrect. The rest of my answers are right, just not the local max.

    Can anyone see where I am going wrong?

    Thanks!
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  2. #2
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    I also get a local max at f(-2) = 161.

    nothing "wrong" that I can see ... online "answer" systems can be a pain-in-the-@$$
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  3. #3
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    thanks, at least I can know that I'm not totally stupid before I email the prof.
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