Foxmaths! 2.0

July 8, 2008

Calculus and Geometry: A Family of Ellipses

Filed under: Maths — Tags: , , , — Fox @ 10:58 pm

The answer to the question never directly stated in the previous post is the following equation.

\frac{ x^2 }{ {(1 - a)}^2 } + \frac{ y^2 }{ a^2 } = 1

Where a varies from 0 to 1.

It’s an interesting equation, so I’m just going to consider it by itself, independent of the problem not really discussed.

Plotting that equation on the xy plane for various values of a (namely, 1/10 to 9/10 at increments of 1/10) produces the following picture.

Family of Ellipses

Some nice symmetries going on there. But notice the way there seems to be a very definite boundary to the shape formed by all the ellipses overlayed. Is there a nice mathematical description of that shape?

Of course, there likely is, or I wouldn’t be talking about it.
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