# Thread: Proof required.

1. ## Proof required.

Let f(x) be a function with n distinct zeros on [a, b]. Prove that, f'(x) has at least n − 1 distinct zeros on [a, b].

Don't really know where to start here, any help is appreciated.

2. But does it say if the function is differentiable in $(a,b)$?

If so, remember Rolle's Theorem, given real numbers $c if $f(x)$ is continuous in $[c,d]$,differentiable in $(c,d)$, and $f(c)=f(d)$ then there is a certain $c$ in $(c,d)$ such that $f'(c)=0$

3. yes the function is differentiable on {a b} and continuous. We need to prove that there is n-1 zeros and not that there is a zero(rolle's) --> f'(c)= 0. For example the function itself may have 5 zeros on [a b] so its derivative will have 4 on [a b], that what is required to prove

4. These are our zeros

Let $x_1,x_2,x_3,...,x_n\in[a,b]$ such that $x_1, and $f(x_i)=0$ $\forall{i\in{N}}$ $1\leq{i}\leq{n}$

Consider $[x_1,x_2]$, $[x_2,x_3]$ ... $[x_{n-1},x_n]$

And apply Rolle's Theorem to them

For example
Since f(x) is continuos in $[x_1,x_2]$, differentiable in $(x_1,x_2)$ and $f(x_1)=f(x_2)=0$ we can assure there's a certain $c_1\in{(x_1,x_2)}$ such that $f'(c_1)=0$

5. Hello, somestudent2!

I must assume that the function is continuous on $[a,b].$

Let $f(x)$ be a function with $n$ distinct zeros on $[a, b].$

Prove that $f'(x)$ has at least $n - 1$ distinct zeros on $[a, b].$

Maybe a sketch will help you visualize the problem . . .

Suppose $f(x)$ has four distinct zeros on $[a,b].$
Code:
        |
|           --*--
|          *     *              --*--
|        *         *           *     *
- - + - + - o - - - - - o - - - - o - - - -o- - - + - -
|   a                  *     *            *   b
|      *                 --*--
|

Then there are at least three horizontal tangents on $[a,b].$

6. thank you very much guys