== Simple formula for the focus position f == 

 I think, the article would benefit from an easy to find and simple formula for the position of the focus as a function of the radii, 
 especially for drawing ellipses with office programs, where the width (=2a) and the height (=2b) are usually given. 
 If I read correctly, then the focus is at  and the eccentricity is .  
 Taking the square of both equations results in a very simple formula:  
 : or after taking the square root the square  
 I found this equation also in the eccentricity subsection. So I'll split this off and make a new subsection named Focus.