Equation of Ellipse in Reduced Form/Cartesian Frame

Theorem
The equation of $K$ is:
 * $\dfrac {x^2} {a^2} + \dfrac {y^2} {b^2} = 1$

Proof

 * EllipseEquation.png

By definition, the foci $F_1$ and $F_2$ of $K$ are located at $\left({-c, 0}\right)$ and $\left({c, 0}\right)$ respectively.

Let the vertices of $K$ be $V_1$ and $V_2$.

By definition, these are located at $\left({-a, 0}\right)$ and $\left({a, 0}\right)$.

Let the covertices of $K$ be $C_1$ and $C_2$.

By definition, these are located at $\left({0, -b}\right)$ and $\left({0, b}\right)$.

Let $P = \left({x, y}\right)$ be an arbitrary point on the locus of $K$.

From the equidistance property of $K$ we have that:


 * $F_1 P + F_2 P = d$

where $d$ is a constant for this particular ellipse.

Taking the particular instance of $P$ to be $V_2 = \left({a, 0}\right)$ it follows that $d = 2 a$.

Also, from Focus of Ellipse from Major and Minor Axis:
 * $a^2 - c^2 = b^2$

Then: