For the expression f(x) = a x2 + bx + c (a > 0), b2 > 4ac always. A real value x0 will lie in between the roots of f(x) if
f(x0) < 0
Here a > 0. So, the graph of f(x) will be in cup shape.
Visualizing the shape through graph,
Let α, β be the roots,
Observe that for any value x0 on x-axis in between α and β, f(x0) < 0
So, f(x0) < 0 is required condition