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Find the area of the region

  1. Jul 18, 2006 #1
    This problem has me stumped because I can't figure out how to find the area of the region. I got an integral, but I don't know how to evaluate it. Here's the problem:

    Find the area of the region bounded by the graph of [tex]y^2 = x^2 - x^4[/tex].

    I solved the equation for y and got [tex]y = \pm\sqrt{x^2 - x^4}[/tex].

    If I graph it, it looks like a bow tie with four symmetrical regions, so I decided to find the area of the top-right region. Here's the integral I came up with:

    [tex]\int_{0}^{1} \sqrt{x^2 - x^4} dx[/tex].

    That's all well and good I guess, but I have no idea how to evaluate that integral! As far as I know, I don't yet have the calculus knowledge to solve that type of integral. Should I solve for x and put the integral in terms of y?
     
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  3. Jul 18, 2006 #2

    HallsofIvy

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    Did you consider factoring:
    [tex]\sqrt{x^2- x^4}= \sqrt{x^2(1- x^2)}= x\sqrt{1- x^2}[/tex]
    Looks like an easy substitution now.
     
  4. Jul 18, 2006 #3
    Duh! That was it! I did that and got an area of 2/3 for one segment, so I multiplied by 4 and got a total area of 8/3 for the whole thing. Does that sound right? Thanks for the help!
     
  5. Jul 19, 2006 #4

    HallsofIvy

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    Did you take into account the factor of 1/2 in [itex]-\frac{1}{2}du= xdx[/itex]?
     
  6. Jul 22, 2006 #5
    Sorry about the delayed response; I was gone for a few days. I kind of rushed through it and must have skipped the factor. So the area would be 4/3 then, correct?
     
  7. Jul 22, 2006 #6

    HallsofIvy

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    [tex]\int_{0}^{1} \sqrt{x^2 - x^4} dx= \int_0^1 \sqrt{1- x^2}x dx[/tex]

    Let [itex]u= 1- x^2[/itex] so [itex]du= -2x dx[/itex] or [itex]-\frac{1}{2}du= x dx[/itex]. When x= 0, u= 1 and when x= 1, u= 0 so the integral becomes
    [tex]-\frac{1}{2}\int_1^0 u^{\frac{1}{2}}du= \int_0^1 u^{\frac{1}{2}}du= \frac{1}{2}\fra{2}{3}u^{\frac{3}{2}\right}_{u=0}^1= \frac{1}{3}[/tex]
    Each segment has area 1/3 so all 4 have area 4/3.
     
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