Math Halp!

Discussion in 'Community Discussion' started by imaketouchtheme, Dec 11, 2012.

  1. imaketouchtheme macrumors 65816

    Dec 5, 2007
    So, after being out of traditional classroom math for over two years now, I'm starting to struggle with basic word problems. I'm taking an online practice exam for my upcoming math class, and I cannot figure out the answer to this question.

    "A rectangular box with volume 360 cubic feet is built with a square base and top. The cost is $1.50 per square foot for the top and the bottom and $2.00 per square foot for the sides. Let x represent the length of a side of the base. Express the cost the box as a function of x."

    Thanks for any help... :)
  2. r.j.s Moderator emeritus


    Mar 7, 2007
  3. dukebound85 macrumors P6


    Jul 17, 2005
    5045 feet above sea level
    wouldn't it be

    2*(1.5x^2)+4*2(x*y) = 360

    where x is the length and width and y is the depth?
  4. leighonigar macrumors 6502a

    May 5, 2007
    In situations like this drawing a picture might help.

    You know that you've got the top and bottom of your box and these are squares of area x times x or x^2. They in total have area 2x^2 and so the cost of the top and bottom is that times $1.5 I.e. 1.5(2x^2) or 3x^2

    The sides are each of area x times the vertical, which I'll call y, and there are four of them, 4(xy), cost 2 a sq ft so 8xy.

    The total cost is $360 which = 3x^2 + 8xy At this point I'm puzzled as this differs from what was suggested first above... Still, I've shown my working so am confident(!)

    I suspect r.j.s. was thinking about volumes in a muddled sort of way?
  5. balamw Moderator


    Staff Member

    Aug 16, 2005
    New England
    O RLY?


    HINT: the volume of the box is a function of x and y which allows you to express y as a function of x. Stick that back in the formula for the cost of the box.

    EDIT: HINT 2: It helps to also write the units down. This way you don't try to equate $ and cubic feet.

  6. leighonigar macrumors 6502a

    May 5, 2007

    Good times for a change! Thanks for that :)
  7. r.j.s Moderator emeritus


    Mar 7, 2007
    I forgot to double and was don't know why I forgot the x for the sides ...

    I shouldn't do maths on morphine.
  8. leighonigar, Dec 11, 2012
    Last edited: Dec 11, 2012

    leighonigar macrumors 6502a

    May 5, 2007
    I wouldn't worry, I misread the question so was at least as muddled... !

    Anyway, it is actually possible now with balamw's insight.

    The formula I derived above is for cost, but we need to get rid of y.

    As the formula for the volume of the box which we know(!) is x.x.y then x^2y = 360

    Rearranging we get a formula for y which we can substitute back in.
  9. iJohnHenry macrumors P6


    Mar 22, 2008
    On tenterhooks
  10. chown33 macrumors 604

    Aug 9, 2009
    Sailing beyond the sunset
    That should be the title of an xkcd cartoon.

    Or a series of cartoons: I shouldn't do ____ on morphine. Kinda like his "My hobby" series.
  11. eric/ Guest


    Sep 19, 2011
    Ohio, United States
    ALWAYS ALWAYS ALWAYS draw a picture. Start there, then brute logic your way through it.

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