Chapter 10 - Parametric and Polar Curves - 10.4 Conic Sections - 10.4 Exercises - Page 750: 2

$$\eqalign{ & {\text{An ellipse is the set of points in a plane whose distances from }} \cr & {\text{two fixed points have a constant sum that we denote }}2a \cr & {\text{Each of the two fixed points is a focus }}({\text{plural foci}}). \cr} $$

$$\eqalign{ & {\text{An ellipse is the set of points in a plane whose distances from }} \cr & {\text{two fixed points have a constant sum that we denote }}2a \cr & {\text{Each of the two fixed points is a focus }}({\text{plural foci}}). \cr} $$