Twice continuously differentiable function

[Jonas Rist]

Hello again,

another problem: given: a function

f:[0,\infty)\rightarrow\mathbb{R},f\in C^2(\mathbb{R}^+,\mathbb{R})\\

The Derivatives

f,f''\\

are bounded.

It is to proof that

\rvert f'(x)\rvert\le\frac{2}{h}\rvert\rvert f\rvert\rvert_{\infty}+\frac{2}{h}\rvert\lvert f''\rvert\rvert_{\infty}\\


\forall x\ge 0,h>0\\

and:

\rvert\rvert f'\rvert\rvert_{\infty}\le 2(\rvert\rvert f\rvert\rvert_{\infty})^{\frac{1}{2}}(\rvert\rvert f''\rvert\rvert_{\infty})^{\frac{1}{2}}\\

I began like this:

f'(x)=\int_{0}^{x}f''(x)dx\Rightarrow

\rvert f'(x)\rvert\le\rvert\int_{0}^{x}f''(x)dx\rvert\le\int_{0}^{x}\rvert f''(x)\rvert dx

But then already I don´t know how to go on
I´d be glad to get some hints!
Thanks
Jonas

EDIT: Would it make sense to apply the Tayler series here?

 

[kuengb]

What is h ?

 

[matt grime]

Don't include x as the variable in your integral and as a limit, it will only confuse you unnecessarily.