What is the area of an ellipse with equation
This ellipse is the image of the unit circle
under the transformation
This transformation maps a square into a rectangle whose area is
times the area of the square.
Unions of disjoint squares inside (or crossing) the circle are mapped into unions of disjoint rectangles inside (or crossing) the ellipse.
The area of the ellipse must therefore be
times the area of the unit circle, or
The Demonstration maps squares with side
into rectangles with sides
and computes inner and outer approximations to the areas of the unit circle and the image ellipse.
The difference between the outer and inner areas for the circle is less than
, and hence the difference between the outer and inner areas for the ellipse is less than
R. B. Kirchner, "The Area of the Ellipse," The American Mathematical Monthly
(7), 1961 p. 653.