Show that the graph of an equation of the form
$$
A x^{2}+C y^{2}+D x+E y+F=0 \quad A \neq 0, C \neq 0
$$
where $A$ and $C$ are of the same sign,
(a) is an ellipse if $\frac{D^{2}}{4 A}+\frac{E^{2}}{4 C}-F$ is the same sign as $A$.
(b) is a point if $\frac{D^{2}}{4 A}+\frac{E^{2}}{4 C}-F=0$.
(c) contains no points if $\frac{D^{2}}{4 A}+\frac{E^{2}}{4 C}-F$ is of opposite sign to $A$.