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Math Help - Calculus : Concavity

  1. #1
    Junior Member
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    Calculus : Concavity

    the problem is y = 2 + 3x - X^3 and i'm supposed to find out for values of X is it concave down.


    when I take the double derivative it comes out to y" = -6x. I set that equal to zero and y" = 0. How do I find out which values of X are concave down? I know the answer is X > 0 but i'm trying to understand WHY. Please help thanks.
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  2. #2
    Math Engineering Student
    Krizalid's Avatar
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    Concave down when y''<0, so y''=-6x<0\implies x>0.
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