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Math Help - university engineering exam paper question on surface area of a box

  1. #1
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    university engineering exam paper question on surface area of a box

    a box with no lid has base rectangle of sides x,y and volume of the box is 9

    i have shown that the surface area, S, is given by

    S=xy+(18/x)+(18/y)

    now i have to find the values of x and y such that S is a minimum but not sure where to start.
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  2. #2
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    Well, start by finding the critical points of S.
    Those points are the solutions of the equations S_x=0 & S_y=0.
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  3. #3
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    that is how i would do it, but the paper is foundation engineering maths and this has not been taught yet.

    doing the Sx= Sy=0 leads to x=y. can this be deduced without resorting to partial diff?
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