For the equation a x2 + bx + c = 0 having 2 real roots, one root lies between real values x1 & x2 if
f( ) f( ) < 0
Let a > 0, graph of f(x) is upward (cup shaped)parabola.
Let α be the root of f(x) between x1 & x2
From graph, f( x1) > 0 and f( x2) < 0
So, f( x1) f( x2) < 0. Similarly, for a<0, the same condition can be proved.