Part of the SVG specification involves creating arcs from a description about an elliptical arc. http://www.w3.org/TR/SVG/paths.html
It species the arc as two points (x0,y0) - the last coordinates from the previous graphics command, and a destination point (x,y). (x0,y0) and (x,y) are on an ellipse. We also know the size/radii of the ellipse rx and ry, and the rotation r of that ellipse.
But we don't know the centre of the ellipse. There are two possible values for this.
How do I determine these two possible centre values. (call it (a,b))
I managed to write two simultaneous quadratic equations in both a and b (and a and b squared). But didn't know how to proceed to solve it.