x²+y²−2yx+2y+2x+1 = 0,  x²−xy+y²/3 = 1,  x²−4xy+y²/3 = 1,  x²−y²+x−y = 0

source("http://macosa.dima.unige.it/r.R")
co1=c(1,-2,1,2,2,1); Conic(co1)
PLANE(-4,1, -4,1); CURVE(C1,"black")
POINT(xF,yF,"brown"); POINT(xV,yV,"red"); CUR(C2,"brown")
#
co2=c(1,-1,1/3,0,0,-1); Conic(co2)
PLANE(-3.5,3.5, -3.5,3.5); CURV(C1,"black"); POINT(xC,yC,"brown")
Point(xF1,yF1,"brown"); Point(xF2,yF2,"brown"); POINT(xC,yC,"red")
#
co3=c(1,-4,1/3,0,0,-1); Conic(co3)
PLANE(-2,2, -2,2); CURV(C1,"black"); CUR(C2,"brown")
POINT(xF1,yF1,"brown"); POINT(xF2,yF2,"brown"); POINT(xC,yC,"red")
#
co4=c(1,0,-1,1,-1,0); Conic(co4)
PLANE(-2,2, -2,2); CURV(C1,"black")