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Let be $f:[0,1]\rightarrow\mathbb{R}$ a continuous function such that far all $a,b\in [0,1]$ with $a<b$

$$f\left(\frac{a+b}{2}\right)\leq\frac{1}{b-a}\int_a^b f(x)\,dx.$$

How to prove that $f$ is a convex function? Thank you for your help.

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put on hold as off-topic by Douglas Zare, Robert Israel, Jeremy Rouse, Stefan Kohl, S. Carnahan 6 hours ago

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1  
When you say "how to prove" - do you mean to say that you've seen this result stated somewhere, but without ptoof? –  Yemon Choi 9 hours ago
    
No need to assume or mention $\ a<b$. –  Włodzimierz Holsztyński 9 hours ago
    
It was asked to me in a written exam, but I wasn't able to prove this fact. I'm thinking about it and it seems to me non trivial. –  Felice Iandoli 9 hours ago
2  
My suggestion: please try a text book on convex functions (e.g., Varberg's book); search google for "hermite-hadamard" inequality –  Suvrit 8 hours ago
1  
Ah, found your question math.stackexchange.com/questions/927824/…. I'll answer it there. –  Robert Israel 6 hours ago

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