p(y-q)+qx=0
p^2+q^2=a^2
(y-q)-pQp+Qpx=0
2p+2qQp=0
Qp=-p/q

(y-q)+p^2/q-xp/q=0
q(y-p)+p^2-px=0
qy-px-pq+p^2=0