Register FAQ / Rules Forum Spy Search Today's Posts Mark Forums Read
 MacRumors Forums Math Halp!
 User Name Remember Me? Password

 Thread Tools Search this Thread Display Modes
 Dec 11, 2012, 06:29 PM #1 imaketouchtheme macrumors 6502a     Join Date: Dec 2007 Location: WV 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, 06:35 PM #2 r.j.s Moderator emeritus     Join Date: Mar 2007 Location: Fort Knox (1.5x^2)(2y)=360 is a good start for you. 0
Dec 11, 2012, 06:41 PM   #3
dukebound85
macrumors P6

Join Date: Jul 2005
Location: 5045 feet above sea level
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?
0
 Dec 11, 2012, 06:49 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, 06:56 PM   #5
balamw
Moderator

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
__________________
MBA (13" 1.7 GHz 128GB), UMBP (15" SD 2.8 GHz), UMB (13" 2.4 GHz), iMac (17" Yonah), 32GB iPad 3 WiFi+LTE, 64 GB iPad WiFi, 32 GB iPhone 5, Airport Extreme
0
Dec 11, 2012, 06:58 PM   #6
leighonigar
macrumors 6502a

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 :-)
0
Dec 11, 2012, 07:06 PM   #7
r.j.s
Moderator emeritus

Join Date: Mar 2007
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.
0
Dec 11, 2012, 07:11 PM   #8
leighonigar
macrumors 6502a

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 07:17 PM.
0
 Dec 11, 2012, 07:16 PM #9 iJohnHenry Banned   Join Date: Mar 2008 Location: On tenterhooks I have 'people' for that type of thing. 0
Dec 11, 2012, 11:10 PM   #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.
0
 Dec 11, 2012, 11:13 PM #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

 Thread Tools Search this Thread Search this Thread: Advanced Search Display Modes Linear Mode

 Similar Threads thread Thread Starter Forum Replies Last Post PyjamasApps iPhone and iPod touch Apps 0 Jul 16, 2013 04:00 AM PyjamasApps iPad Apps 0 Jul 16, 2013 03:56 AM klkatielong iPad Apps 0 Jul 18, 2012 12:56 PM Gaea deBomb iPhone and iPod touch Apps 0 Jul 3, 2012 04:35 AM mobile.esd.mx iPhone and iPod touch Apps 0 Jun 1, 2012 04:17 PM

 Forum Jump User Control Panel Private Messages Subscriptions Who's Online Search Forums Forums Home News and Article Discussion     MacRumors.com News Discussion     Mac Blog Discussion     iOS Blog Discussion iPhone, iPod and iPad     iOS         iOS 8         iOS 7         iOS 6         iOS 5 and earlier         Jailbreaks and iOS Hacks     iPhone         iPhone Tips, Help and Troubleshooting         iPhone Accessories         iPhone Launch Meetups         iPhone Wallpapers etc.     iPad         iPad Tips, Help and Troubleshooting         iPad Accessories         iPad Launch Meetups         iPad Wallpapers etc.         iPad Apps     iPod touch         iPod touch Accessories     iPod     Alternatives to iOS and iOS Devices Apple Applications     iPhone and iPod touch Apps     iPad Apps     Mac Applications and Mac App Store Apple Hardware     Desktops         iMac         Mac Pro         Mac mini     Notebooks         MacBook         MacBook Pro         MacBook Air     PowerPC Macs     Apple TV and Home Theater     Mac Peripherals     Buying Tips and Advice Apple Systems and Services     Mac Basics and Help     OS X         OS X Yosemite (10.10)         OS X Mavericks (10.9)         OS X 10.8 Mountain Lion         Mac OS X 10.7 Lion         Mac OS X Server, Xserve, and Networking     iCloud and Apple Services     Windows, Linux & Others on the Mac     Programming         iPhone/iPad Programming         Mac Programming         App Store Business, Legal and Marketıng Special Interests     Mac and PC Games         Console Games     Digital Audio     Visual Media         Design and Graphics         Digital Photography         Digital Video         Web Design and Development     Apple Collectors     Distributed Computing Mac Community     Community Discussion         Apple, Industry and Internet Discussion         Current Events         Politics, Religion, Social Issues     Picture Gallery         UI Customization     Site and Forum Feedback         Mac Guides Private Forums Archive     Wasteland     Archives of Old Posts         MacRumors News Discussion (archive)         MacBytes.com News Discussion         Buying Tips, Advice and Discussion (archive)         Community         Daily Tunes Site Discussion and Feedback         Event Archives             Macworld San Francisco 2008         Games         General Mac Discussion         Hardware Rumors         iPhone Purchaser Meetups         Leopard Event Meetups         Mac Help/Tips         Mac OS X 10.3 (Panther) Discussion         Mac Scene         Macintosh Computers         MacRumors Old Skool         Marketplace Archive 1 (Posts count)         Marketplace Archive 2             iPhone Marketplace Archive         Music Discussion         New Mac Application Announcements         Product Recommendations/Reviews         Site News         Switch Stories         Web Design and Development (archive)         Past Contests             1,000,000 Post Contest             2,000,000 Post Contest             3,000,000 Post Contest             4,000,000 Post Contest             5,000,000 Post Contest             Ten Million Post Contest

All times are GMT -5. The time now is 01:14 PM.

Mac Rumors | Mac | iPhone | iPhone Game Reviews | iPhone Apps
 Contact Us - MacRumors Forums - Archive - Privacy Statement / DMCA Agent - Top

Mobile Version | Fixed | Fluid | Fluid HD
Copyright 2002-2013, MacRumors.com, LLC