Double integrals

  • Thread starter haroldholt
  • Start date
  • #1
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I'm having some trouble with this particular question.

∫∫x dA bound by y = 4x^3 - x^4 and y = 3 - 4x + 4x^2.

All I can think to do is equate the two equations to find where they intercept to give the bounds for the double integral giving 0 = x^4 - 4x^3 + 4x^2 - 4x + 3. But I don't know where to go from here.

Any help would be appreciated.
 

Answers and Replies

  • #2
That's pretty nasty.
Maybe you could try Newton's method? Take an initial guess, e.g. x=1...
Actually, it looks like x=1 works.. (1 - 4 + 4 - 4 + 3 = -3 + 4 - 4 + 3 = 0)
So since we have that, divide through by (x-1) to get the other roots.
Have fun. :D
 
  • #3
NateTG
Science Advisor
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The only possible rational roots are:
[tex]\pm 1, \pm 3[/tex]
so you could start by checking whether those are intersections.
 
  • #4
21
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What method did you use to find those roots?
 
  • #6
21
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Cheers mate. Can't say I've ever heard of the rational zero theorem.
 

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