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Steel pipe around a hallway

  1. Apr 24, 2013 #1
    1. The problem statement, all variables and given/known data

    0tBzblE.png

    hBWBwO3.png

    2. Relevant equations

    Trigonometric identities and differentiation

    3. The attempt at a solution

    It's pretty simple to solve this question when the hallway is a right angle.

    G3GFWTB.jpg

    Differentiating -6cscθ + 9secθ and setting it equal to zero and solving will yield the proper answer. But I don't know if the same method can be applied for this question? How would I start to solve this question?
     
  2. jcsd
  3. Apr 24, 2013 #2

    Office_Shredder

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    The same method can, but not using the same function obviously. Do you understand where the equation -6cscθ + 9secθ comes from geometrically? The same geometric argument can construct a function for this other hallway that you need to minimize
     
  4. Apr 24, 2013 #3
    For the right-angle turn, you look at the part of the pipe in each hallway, and add them together. So cosθ = 9/L1 and -sinθ = 6/L2. You can make similar triangles in the hallway of question, but I don't think θ is can be used as the angle for both triangles. And how does the pi/4 play a part in this?
     
  5. Apr 24, 2013 #4

    Office_Shredder

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    Let θ be the angle that the pipe makes with the 12 foot hallway's wall. You'll have to do a little geometry (not much, just tracking some angles) to find out what angle the pipe makes with the 8 foot hallway's wall, then set up a similar equation to calculate how long the pipe is. The fact that we have an extra pi/4 angle is just going to change what the relationship between the angle with the 12 foot wall and the angle with the 8 foot wall are related
     
  6. Apr 25, 2013 #5
    So this is what I came up with,

    CvC6bLS.png

    but the resulting theta values do not equate to the answer, which is 51.7762 ft.
     
  7. Apr 25, 2013 #6

    Office_Shredder

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    Adding up all the angles on one half of the red line on the right should give me a total of [itex] \pi [/itex]. But on the right half of the line you have
    [tex]\pi/4 + \pi/4+\pi/4+\pi/4+\theta+\pi/2-\theta = 3\pi/2[/tex]

    So you have to double check those
     
  8. Apr 26, 2013 #7
    Got it! It was θ - π/4 for the second triangle. Thanks :).
     
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