# Proof of equation of ellipse

The ellipse can be defined as a conic section with eccentricity lesser than unity. How can you derive the equation of the ellipse using this definition? I can't find proof of the formula using this definition alone.

• With the help of Dandelin Spheres, the section-of-a-cone definition easily gives rise to the sum-of-distances-from-two-points-is-a-constant definition. – Blue Dec 24 '15 at 19:30