Integral of function over ellipse

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Discussion Overview

The discussion revolves around the integral of a function over the surface of an ellipse, specifically the expression \(\iint_S \sqrt{1-\left(\frac{x}{a}\right)^2 -\left(\frac{y}{b}\right)^2} dS\), where \(S\) represents the surface of the ellipse defined by \(\left(\frac{x}{a}\right)^2+\left(\frac{y}{b}\right)^2 = 1\). The scope includes mathematical reasoning and potential methods for evaluating the integral.

Discussion Character

  • Mathematical reasoning
  • Exploratory

Main Points Raised

  • Nick seeks assistance in evaluating the integral over the ellipse's surface.
  • One participant questions whether the integral pertains to the interior of the ellipse and suggests using Green's Theorem, proposing a substitution related to the ellipse's boundary.
  • Another participant proposes a change of coordinates involving a Jacobian determinant, leading to a reformulation of the integral in terms of a unit circle.
  • A further suggestion involves a specific substitution using polar coordinates with respect to the ellipse's dimensions.

Areas of Agreement / Disagreement

Participants have not reached a consensus on the best approach to evaluate the integral, and multiple methods and interpretations are presented without resolution.

Contextual Notes

The discussion does not clarify whether the integral is intended for the interior or the boundary of the ellipse, and the implications of different coordinate transformations are not fully explored.

nickthequick
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Hi,

I'm trying to find
\iint_S \sqrt{1-\left(\frac{x}{a}\right)^2 -\left(\frac{y}{b}\right)^2} dS

where S is the surface of an ellipse with boundary given by \left(\frac{x}{a}\right)^2+\left(\frac{y}{b}\right)^2 = 1.

Any suggestions are appreciated!

Thanks,

Nick
 
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Do you mean the interior of an ellipse?

Anyway, the first thing I though of is Green's Theorem for some reason. Probably since then we can make the substitution ##\left(\dfrac xa\right)^2+\left(\dfrac yb\right)^2=1##.

The second thing I thought of was a change of coordinates and a multiplication by the Jacobian determinant, then we have it reduced to

$$a\cdot b\cdot\iint_C\sqrt{1-m^2-n^2}\mathrm{d}S'$$

where C is the unit circle wrt m and n and S' should have a fairly obvious definition.
 
Last edited:
Try the substitution$$
\frac x a = r\cos\theta,\,\frac y b = r\sin\theta$$
 
Got it!

Thanks
 

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