Definition: A
real polynomial $f(x,y,z)$ is homogeneous of degree 2 if
$f(tx,ty,tz) = t^2 f(x,y,z)$ for all $x,y,z$ and $t \ne 0$.
Suppose $P(x,y)=Ax^2 + Bxy + Cy^2 + Dx + Ey + F$ and
$f(x,y,z)= Ax^2 + Bxy + Cy^2 + Dxz + Eyz + Fz^2$ is the
corresponding homogeneous polynomial of degree 2.
Fact: $P(x,y) = f(x,y,1)$ so
$P(x,y) = 0$ if and
only if $ f(x,y,1) = 0$.