The Darboux method introduces algebraic solutions quite useful to obtain in
variant and first integrals of polynomial differential systems. Here we stu
dy the 2D Lotka-Volterra (LVS) and the complex quadratic system (QS) using
straight lines for both and conics for the LVS. The conditions needed to ob
tain these invariants are given and a study of the phase space portrait is
done. (C) 1999 American Institute of Physics. [S0022-2488(99)02604-3].