Build & Fit - Ellipse
An ellipse can be defined symbolically by parameters a,b or by location of two foci and a point. What are the relations between these representations?
In the left panel you can define a blue ellipse by varying the values of the a and b sliders. In the right panel you define the green ellipse by adjusting the location of the foci and a point on the green ellipse. Now try to vary the green ellipse to match the blue one.
Alteratively, start by defining a green ellipse in right panel, and see if you can adjust the a and b parameters in the left panel so that the blue ellipse matches the green one.