theorem Th16:
  z1 <> 0 & Polynom(z1,z2,z) = 0 implies z = -(z2/z1)
proof
  assume z1 <> 0 & Polynom(z1,z2,z) = 0;
  then z = (-z2)*z1" by XCMPLX_1:203;
  hence thesis;
end;
