Intersection of Plane with Sphere is Circle

Theorem
The intersection of a plane with a sphere is a circle.

Proof
Let $S$ be a sphere of radius $R$ whose center is located for convenience at the origin.

Let $P$ be a plane which intersects $S$ but is not a tangent plane to $S$.

It is to be shown that $S \cap P$ is a circle.

Let $S$ and $P$ be embedded in a (real) cartesian space of $3$ dimensions.

Let this space be rotated until $P$ is parallel to the plane $z = 0$.

Thus from the Equation of Plane we have that $P$ can be described as:
 * $z = c$

where $\size c < R$ (or $P$ would not intersect $S$).

Let $A = \tuple {x, y, z}$ be an arbitrary point on $S \cap P$.

We have that:

The result follows from Equation of Circle.