Why there are no brackets in $e^x$ or $\ln x$? I have seen that one uses often brackets when one evaluates functions, like $f(3)=5$, never $f3=5$. But why some textbooks makes it clear that it is okay to write $\ln 3$ or $e^3$ instead of $\ln (3)$ or $e^{(3)}$? Why do $\ln, e, \sin,\cos,\tan\ldots$ uses different rules of bracketing compared to $f,g,h\ldots$? 

Because $e^x, \log x, \sin x$ are all used so often that having to repeatedly use brackets would be annoying. There is also very little chance of confusion. It is basically the same reason why $\log, \sin$ etc have their own latex code...
