For the expression f(x) = a x2 + bx + c (a < 0), b2 > 4ac always. A real value x0 will lie in between the real roots of f(x) if
f() > 0
As a < 0, graph of f(x) will be downward parabola .
Let us visualize through the graph
Let α, β be the roots.
If we take any value x0 on x-axis in between α & β, from the graph we can observe that f(x0) > 0
So, f(x0) > 0 is the condition required.