Link between polynomial and derivative of polynomial I can't seem to solve this problem, can anyone help me please? The problem is: 
Let real numbers $a$,$b$ and $c$, with $a ≤ b ≤ c$ be the 3 roots of the polynomial $p(x)=x^3 + qx^2 + rx + s$. Show that if we divide the interval $[b, c]$ into six equal parts, then one of the root of $p'(x)$ (the derivative of the polynomial $p(x)$) will be in the 4th part.
What I did was:
Because we know the roots of $p(x)$ are $a$,$b$ and $c$, we can write the polynomial $p(x)$ like this: $p(x) = (x-a)(x-b)(x-c)$
So we have $p(x) = x^3 + qx^2 + rx + s = (x-a)(x-b)(x-c)$
We find the value of $q$,$r$ and $s$: 
$q = -(a+b+c)$
$r = (ab + ac + bc)$
$s = -abc$
We have that $p'(x) = 3x^2 + 2qx + r$
We want to find the roots of $p'(x)$, so if we apply the quadratic formula, we get:
$(-2q ± 2*\sqrt{q^2 - 3r})/6$
Because we know the value of q,r and s, we can rewrite this expression like this:
$(2(a+b+c) ± 2\sqrt{a^2 + b^2 + c^2 - ab - bc -ca})/6$
But I am stuck here, any help would be great. Thank you in advance. 

I have an inelegant brute force solution. Since the problem is translation invariant, we may assume $a=0$ to simplify calculations so that $0\leq b\leq c$. It should be clear that the root of $p'(x)$ we are interested in is the largest of the two i.e. the one with the plus sign. We need to prove that
$$\dfrac{3b+3c}{6}\leq\dfrac{2(b+c)+2\sqrt{b^2+c^2-bc}}{6}\leq\dfrac{2b+4c}{6}$$
On the left we obtain
$$b+c\leq 2\sqrt{b^2+c^2-bc}$$
where both sides are non-negative, squaring gives
$$b^2+c^2+2bc\leq 4(b^2+c^2-bc)\Rightarrow 3b^2+3c^2-6bc\geq 0$$
which holds. On the right we have
$$2\sqrt{b^2+c^2-bc}\leq 2c$$
so that
$$b^2+c^2-bc\leq c^2\Rightarrow b(b-c)\leq 0$$
which holds since $0\leq b\leq c$.
