a + ka₁ + (b + kb₁) x + (c + kc₁) y + u₂ + u₃ = 0;
one, and only one, value of k makes the absolute term vanish; hence
one, and only one, curve of the system passes through the origin,
which may be any point in the plane. Make the equation of the system
homogeneous by means of z, and differentiate twice with respect to z;
we obtain thus the equations of the polar conics and polar lines of the
origin with respect to the system.