We have 2 functions

$\displaystyle f_{1}(x,y,z)=A_{1}xyz+B_{1}xy+C_{1}xz+D_{1}yz+E_{1 }x+G_{1}y+

M_{1}z+N_{1}$

$\displaystyle f_{2}(x,y,z)=A_{2}xyz+B_{2}xy+C_{2}xz+D_{2}yz+E_{2 }x+G_{2}y+

M_{2}z+N_{2}$

So i need to find where $\displaystyle f_{1}*f_{2}<0$

Any ideas or mb u know where i can find solution?