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 Dec 11, 2012, 07:29 PM #1 imaketouchtheme macrumors 6502a     Join Date: Dec 2007 Location: West Virginia Math Halp! 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... __________________ Unconventional Hacker 0
 Dec 11, 2012, 07:35 PM #2 r.j.s Demi-God (Moderator emeritus)     Join Date: Mar 2007 Location: Fort Knox (1.5x^2)(2y)=360 is a good start for you. __________________ After the Army: no one can stay forever 0
Dec 11, 2012, 07:41 PM   #3
dukebound85
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Join Date: Jul 2005
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Quote:
 Originally Posted by r.j.s (1.5x^2)(2y)=360 is a good start for you.
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?
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 Dec 11, 2012, 07:49 PM #4 leighonigar macrumors 6502a   Join Date: May 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? 0
Dec 11, 2012, 07:56 PM   #5
balamw
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Join Date: Aug 2005
Location: New England, USA
Quote:
 Originally Posted by leighonigar The total cost is \$360
O RLY?

Quote:
 Originally Posted by imaketouchtheme A rectangular box with volume 360 cubic feet
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.

B
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Dec 11, 2012, 07:58 PM   #6
leighonigar
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Join Date: May 2007
Quote:
 Originally Posted by balamw 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. B

Good times for a change! Thanks for that :-)
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Dec 11, 2012, 08:06 PM   #7
r.j.s
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Location: Fort Knox
Quote:
 Originally Posted by leighonigar I suspect r.j.s. was thinking about volumes in a muddled sort of way?
I forgot to double and was don't know why I forgot the x for the sides ...

I shouldn't do maths on morphine.
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Dec 11, 2012, 08:11 PM   #8
leighonigar
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Join Date: May 2007
Quote:
 Originally Posted by r.j.s I forgot to double and was don't know why I forgot the x for the sides ... I shouldn't do maths on morphine.
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.

Last edited by leighonigar; Dec 11, 2012 at 08:17 PM.
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 Dec 11, 2012, 08:16 PM #9 iJohnHenry macrumors P6     Join Date: Mar 2008 Location: I was never here. Remember that!! I have 'people' for that type of thing. __________________ Never argue with idiots. They'll drag you down to their level, then beat you with experience. 0
Dec 12, 2012, 12:10 AM   #10
chown33
macrumors 603

Join Date: Aug 2009
Quote:
 Originally Posted by r.j.s I shouldn't do maths on morphine.
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.
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 Dec 12, 2012, 12:13 AM #11 eric/ Guest   Join Date: Sep 2011 Location: Ohio, United States ALWAYS ALWAYS ALWAYS draw a picture. Start there, then brute logic your way through it. 0