for b, c, d being Real
for z being Complex st b <> 0 & delta (b,c,d) < 0 & Polynom (0,b,c,d,z) = 0 & not z = (- (c / (2 * b))) + (((sqrt (- (delta (b,c,d)))) / (2 * b)) * <i>) holds
z = (- (c / (2 * b))) + ((- ((sqrt (- (delta (b,c,d)))) / (2 * b))) * <i>)