reserve x, a, b, c for Real;

theorem
  a < 0 & delta(a,b,c) < 0 implies a * x^2 + b * x + c < 0
proof
  assume that
A1: a < 0 and
A2: delta(a,b,c) < 0;
  - delta(a,b,c) > 0 & 4 * a < 0 by A1,A2,XREAL_1:58,132;
  then (- delta(a,b,c))/(4 * a) < 0 by XREAL_1:142;
  then - delta(a,b,c)/(4 * a) < 0 by XCMPLX_1:187;
  then
A3: a * (x + b/(2 * a))^2 + - delta(a,b,c)/(4 * a) < a * (x + b/(2 * a)) ^2
  + 0 by XREAL_1:6;
  a * (x +b/(2 * a))^2 <= 0 by A1,XREAL_1:63,131;
  then a * (x + b/(2 * a))^2 - delta(a,b,c)/(4 * a) < 0 by A3,XXREAL_0:2;
  hence thesis by A1,Th1;
end;
