Maximize Area

  • Thread starter joemama69
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  • #1
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Homework Statement



Note the picture

Homework Equations





The Attempt at a Solution



A of both trianles = xsinQ[tex]\sqrt{x^2 - x^2(sinQ})^2[/tex]

A of rectangle = (w-2x)(xsinQ)

Total A = (w-2x)(xsinQ) + xsinQ[tex]\sqrt{x^2 - x^2(sinQ)^2}[/tex]

A = wxsinQ + 2x2sinQ + x2sinQcosQ

Ax = wsinQ + 4xsinQ + 2xsinQcosQ = 0

-wsinQ = x(4sinQ + 2sinQcosQ)

x = -w/(4 + 2cosQ)

AQ = wxcosQ + 2x2cosQ + x2(sinQ)2 - x2(cosQ)2 = 0

It gets pretty messy when i plug in x, are there anny errors so far
 

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  • #2
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anybody, can i get some help
 
  • #3
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Homework Statement



Note the picture

Homework Equations





The Attempt at a Solution



A of both trianles = xsinQ[tex]\sqrt{x^2 - x^2(sinQ})^2[/tex]

A of rectangle = (w-2x)(xsinQ)

Total A = (w-2x)(xsinQ) + xsinQ[tex]\sqrt{x^2 - x^2(sinQ)^2}[/tex]

A = wxsinQ + 2x2sinQ + x2sinQcosQ

Ax = wsinQ + 4xsinQ + 2xsinQcosQ = 0

-wsinQ = x(4sinQ + 2sinQcosQ)

x = -w/(4 + 2cosQ)

AQ = wxcosQ + 2x2cosQ + x2(sinQ)2 - x2(cosQ)2 = 0

It gets pretty messy when i plug in x, are there anny errors so far
What's the problem you are trying to solve? The picture doesn't give me any idea of what you're trying to do.
 
  • #4
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you must Maximize the area, note the title
 
  • #5
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Some context would have been nice, such as that the area is the cross-sectional area of what looks to be a trough, formed from sheet metal that is W units wide.

Your picture doesn't convey that information, which would have been helpful.

You have a sign error in this line:
A = wxsinQ + 2x2sinQ + x2sinQcosQ
The first '+' sign should be '-'. That makes Ax = WxsinQ - 4xsinQ + 2xsinQcosQ = sinQ[W -4x + 2xcosQ].

The sign error also affects AQ, which I get as xWcosQ -2x2cosQ + x2cos2Q - x2sin2Q.
About the only thing I can think of to do is to factor x out of each term in this partial.

The critical points are where Q = 0 or when x = 0, either of which gives you an area of 0, and whatever you get from some messy equations that remain. One of these is W - 4x + 2xcosQ = 0, which you can solve for x in terms of Q (and W), or solve for Q in terms of x (and W). Whichever variable you solve for, substitute that in the other equation that comes from AQ.
 
  • #6
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Ya, this is pretty much where i ran into problems, i pluged in my x = w/(4-2cosQ) into A sub Q

AQ = 0 = (w2cos)/(4-2cos) - 2w2cos/(4-2cos)2 + (2w2cos2)/(4-2cos)2 - (w2sin2)/(4-2cos2)

I would have to solve that for Q (i got lazy and didnt type them, just assume they are there), which means (tell me if im wrong), i only have to solve the numerators of the expression for 0 in terms of w

I got Q = 0 & 2pi which would make the gutter flat, what happened
 
  • #7
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I didn't work it through that far, so I'll assume that the work you have in post 6 is correct. You can't just "solve the numerators," as you put it, since not all of your expressions have the same denominator; the first has a denominator of 4 - 2cosQ. You'll need to multiply that term by (4 - 2cosQ)/(4 - 2cosQ), and then you can factor out 1/(4 - 2cosQ)2 from all four terms. Finally, you'll need to see what values of Q (theta) make AQ equal to 0.

As I said before, Q = 0 and x = 0 are critical values, but they produce a trough with cross-sectional area 0, so they are not the values you want.
 
  • #8
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ok i was able to factor out the w^{2} and the (4-2cos)^{2} and I got

4cos - 2cos^{2} - 2cos + cos^{2} - sin^{2} = 2cos - con^{2} - sin^{2} = 0


I got Q = pi/3, 5pi/3, 7pi/3

how do i calc which one is the maximum, there are many possibilities
 
  • #9
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Due to the fact that you're working with a physical object, a trough, there is a natural range of values for Q (theta). That should help you eliminate many of the possibilities. If Axx < 0 and AQQ < 0, you're at a local maximum.
 

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