Here is the simple question.(adsbygoogle = window.adsbygoogle || []).push({});

When I differentiate the area of a circle, I got the parameter. As the Area is the sum of Circumference.

But why cannot I get the circumference of ellipse by differentiate the area with respect to the radius (I set it as the longer side). The answer I got is 2πa√1-e^2, where e is eccentricity and a is half the longer side.

THank you!

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# Circumference of ellipse

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