<s>[INST] 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. 
 [/INST] Since $p(x) = (x-a)(x-b)(x-c)$, 
$$\begin{align}p'(x) &= (x-b)(x-c)+(x-a)(x-c)+(x-a)(x-b)\\
        &= (x-b)(x-c) + (x-a)(2x - (b+c))\end{align}$$
At $x = \frac{b+c}{2}$, we have
$$p'(x) = \frac{c-b}{2}\cdot\frac{b-c}{2} + (x-a)\cdot 0 =  -\frac{(c-b)^2}{4} \le 0$$
At $x = \frac{b+2c}{3}$, we have
$$\begin{align}p'(x) &= \frac{2(c-b)}{3}\cdot\frac{b-c}{3}
+ \left(\frac{b+2c}{3} - a\right)\cdot\frac{c-b}{3}\\
&= -\frac{2(c-b)^2}{9} + \left(\frac{2(c-b)}{3} + (b-a)\right)\cdot\frac{c-b}{3}\\
&= \frac{(b-a)(c-b)}{3}\\&\ge 0\end{align}$$
This implies $p'(x)$ has a root in $\left[\frac{b+c}{2},\frac{b+2c}{3}\right]$.
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