The condition that the line xp+yp=1 o be tangent to xzaz+yzbz=1 is
azpz−bzqz = 1
azqz−bzpz = 1
xp+yp=1 xzaz−yzbz=1
yp=−xp+1
y = −qpx+q
q2=a2qzpz−b2
⇒ a2q2−b2p2=p2q2
⇒ a2p2−b2q2 = 1