theorem Th10: :: PASCAL:10

for a, b, c, d, e, f being Real
for u1, u2 being non zero Element of (TOP-REAL 3) st Dir u1 = Dir u2 & qfconic (a,b,c,d,e,f,u1) = 0 holds
qfconic (a,b,c,d,e,f,u2) = 0