Equation of Confocal Conics/Formulation 2

Definition
The equation:
 * $(1): \quad \dfrac {x^2} {a^2} + \dfrac {y^2} {a^2 - c^2} = 1$

where:
 * $\tuple {x, y}$ denotes an arbitrary point in the cartesian plane
 * $c$ is a (strictly) positive constant
 * $a$ is a (strictly) positive parameter

defines the set of all confocal conics whose foci are at $\tuple {\pm c, 0}$.

Proof
Let $a > c$.

Then from Equation of Confocal Ellipses: Formulation 2, $(1)$ defines the set of all confocal ellipses whose foci are at $\tuple {\pm c, 0}$.

Let $a < c$.

Then from Equation of Confocal Hyperbolas, $(1)$ defines the set of all confocal hyperbolas whose foci are at $\tuple {\pm c, 0}$.

Hence the result.

Also see

 * Equation of Confocal Conics/Formulation 1