How to find $f(15/2)$ if $f(x+1)=f(x)$? Suppose $f(x+1)=f(x)$ for all real $x$. If $f$ is a polynomial and $f(5)=11$, then $f(15/2)$ is ??
How do I approach questions like this?

The problem is that how a polynomial can be periodic! Is there any other way than being a constant? So your function is really $f(x)=11$. That's all.
