# Math Help - Differentiable function problem

1. ## Differentiable function problem

$f$ is a differentiable function at $[a,\infty)$.
i need to prove that:
if there is a constant $m>0$ which maintains that $f'(x)\geq m$ to every $x\in[a,\infty),$
so $lim_{x\rightarrow\infty}f(x)=\infty$... thx!!

2. ## Re: Differentiable function problem

Originally Posted by orir
$f$ is a differentiable function on $[a,\infty)$.
i need to prove that:
if there is a constant $m>0$ which maintains that $f'(x)\geq m$ to every $x\in[a,\infty),$
so $lim_{x\rightarrow\infty}f(x)=\infty$... thx!!

If $x>a$ then $\exists x'\in(a,x)$ such that $\frac{f(x)-f(a)}{x-a}=f'(x')\ge m$. WHY?

That means that $f(x)\ge m(x-a)+f(a)$. What can you do with that?

3. ## Re: Differentiable function problem

Originally Posted by Plato
If $x>a$ then $\exists x'\in(a,x)$ such that $\frac{f(x)-f(a)}{x-a}=f'(x')\ge m$. WHY?

That means that $f(x)\ge m(x-a)+f(a)$. What can you do with that?
can i say that $\varepsilon=m(x-a)+f(a)$? so then, $f(x)\ge\varepsilon$??

4. ## Re: Differentiable function problem

Originally Posted by orir
can i say that $\varepsilon=m(x-a)+f(a)$? so then, $f(x)\ge\varepsilon$??
Well, you did not answer the question WHY?

If you want further help, then you must explain where the inequality comes from.

Moreover, what does it mean to say that ${\lim _{x \to \infty }}f(x) = \infty~?$ Give the precise definition.

5. ## Re: Differentiable function problem

oh, i know why. it's because the mean value theorem.
and.. the precise definition is that there is an m>0, and n>0, so that for every x larger than m, f(x) larger than n.

6. ## Re: Differentiable function problem

Originally Posted by orir
oh, i know why. it's because the mean value theorem.
and.. the precise definition is that there is an m>0, and n>0, so that for every x larger than m, f(x) larger than n.

Can you show that ${\lim _{x \to \infty }}\left( {m(x - a) + f(a)} \right) = \infty ~?$

If so, what does that imply?

7. ## Re: Differentiable function problem

i guess i can show that. but i don't know what does that imply...
and what about my way? to say that $n=m(x - a) + f(a)$ and that way show the right of the definition above.

8. ## Re: Differentiable function problem

Originally Posted by orir
i guess i can show that. but i don't know what does that imply...
and what about my way? to say that $n=m(x - a) + f(a)$ and that way show the right of the definition above.

Well $\forall x>a$ we have shown that $f(x)\ge m(x-a)+f(a)\to\infty~.$

9. ## Re: Differentiable function problem

oh, i got you..
but still - does my way work too?