let three points be p,q,r
their coordinates are t1,t2,t3
slope of qr is -1t2t3
thus slope of PS IS t2t3
EQUATION OF PS IS t1y-C=xt1t2t3-ct21t2t3
similarily EQUATION OF QT IS t2y-C=xt1t2t3-ct1t22t3
solving these two we get orthocentre as (-ct1t2t3,-ct1t2t3) which lies on xy=c2