I am solving a Quantum Mechanics problem and I came across these Taylor's series.
So I was wondering, if I have simplified them correctly or is there some other option to simplify these series? I would be extremely grateful, if someone could help, as I am not mathematician myself.
In a general manner, if you consider $f(x+a x)$ for small value of $a$, let $ax=h$ to have $$f(x+h)=f(x)+h f'(x)+\frac 12 h^2 f''(x)+\cdots$$ Now replace $h$ by $a x$ to get what you wrote which is correct.