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Spot the error! (circumference of ellipse)

  1. Jan 10, 2012 #1
    Spot the error:

    The circumference of an ellipse with semi-major axis = 2 and semi-minor axis = 1 (i.e. [itex] \left( \frac{x}{2} \right)^2 + y^2 = 1 [/itex]) is [itex]4 \pi[/itex]. Proof:

    [itex]\textrm{circumference} = \iint_{\mathbb R^2}{ \delta \left( \left( \frac{x}{2} \right)^2 + y^2 - 1 \right) } \mathrm d x \mathrm d y = 2 \iint_{\mathbb R^2}{ \delta \left( \left( \frac{x}{2} \right)^2 + y^2 - 1 \right) } \mathrm d \frac{x}{2} \mathrm d y = 2 \iint_{\mathbb R^2}{ \delta \left( x^2 + y^2 - 1 \right) } \mathrm d x \mathrm d y = 2 \times 2 \pi = 4 \pi [/itex]

    (I couldn't post this in the Math forum since they would just yell at me for using the dirac delta, and it's also not homework as I know the answer.)
  2. jcsd
  3. Jan 10, 2012 #2
    I could not see the mistake yet.. Interesting way to calculate the circumference :)
  4. Jan 10, 2012 #3
    Well, I just moved it to the math forum. So prepare for some yelling :biggrin:
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