A little problem

  • Thread starter Sombra
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  • #1
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Easy problem... but urgent!

y = (x^5 + 4x^4)^(1/2)

They want me to find the area of this loop, and the boundaries are -4 and 0, but when I integrate it and plug in the -4, I get 0, which is clearly not the case. If you can help, I would love it.
 
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Answers and Replies

  • #2
Tide
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I think you left out the second limit on the integration!
 
  • #3
WaR
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Can you post your integrating steps?
That integration is not equal to [tex]0[/tex]
But
[tex]\int_{-4}^{0} \sqrt{x^{5}+4x^{4}} dx = \frac{2048}{105}[/tex]
 
  • #4
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I did:

Integration of (x^5 +4x^4)^(1/2) is 2/3 (x^5 +4x^4)^(3/2)

So 2/3(0 + 0)^(3/2) = 0

0 - [2/3 (-1024 + 1024)^(3/2) = 0

It doesn't work.
 
  • #5
Tide
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No, you integrated incorrectly!
 
  • #6
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ok, the answer given in the back of the book is 4096/105. I know that even if that answer is not correct, it is more correct than mine because the area of that loop in the graph is clearly not 0.
 
  • #7
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ok, I think the book is wrong because the calculator said that it was 19.50476233. I still don't know how this works though because.... I still get 0.
 
  • #8
Pyrrhus
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Little hint

[tex] \int_{-4}^{0} \sqrt{x^{5}+4x^{4}} dx [/tex]

[tex] \int_{-4}^{0} x^{2} \sqrt{x+4} dx [/tex]
 
  • #9
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that's how the original problem was set up. I distributed, thinking it would be easier. In any case, I get 0. Thanks
 
  • #10
Pyrrhus
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Post the original problem as the book states it.
 
  • #11
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Sketch the graph y^2 = x^4(x+4) and find the area enclosed by the loop.
 
  • #12
Tide
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That is distinctly different from the problem you first posed - but your integration is still incorrect! :-)
 
  • #13
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the boundaries are still -4 and 0, and my answer still comes out to be 0. What is correct? I guess I am not understanding. Please help!!
 
  • #14
Tide
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[tex]\int x^2 \sqrt {x+4} dx = \frac {2 (x+4)^{3/2} (15 x^2 -48 x + 128)}{105}[/tex]
 
  • #15
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ok so 2/3(x+4)^3/2, I got that part, but how did you get the other part. I am not familiar with integrating something undistributed unless it follows the f(x) * f'(x) rule. Sorry for all the questions. Thank you so much!!
 
  • #16
futb0l
Use cyclovenom's hint
 
  • #17
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Now I am totally lost.... I know I have to integrate only the top half of this loop and multiply it by 2 to find the area or it will equal 0. I just do not know how to set up the problem to make it only the top half of the loop.
 
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  • #18
Tide
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You can integrate by parts. You may get a slightly different form than I did because I did some simplification.
 

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