Let f : R ® R be such that for all x Î R, (21+x + 21–x ), f (x) and (3x + 3–x ) are in A.P., then the minimum value of f (x) is:

Let $f: R \rightarrow R$ be such that for all $x \in R,\left(2^{1+x}+2^{1-x}\right), f(x)$ and $\left(3^{x}+3^{-x}\right)$ are in A.P., then the minimum value of $f(x)$ is: