[QUOTE=alteraus;794113]Hi there, I am trying to find the Maclaurin expansion of 3rd degree of the function but I get strange result ..

I need to ask what am I doing wrong ..

I started with

where for there holds ...

This is, at least, one problem: the first two terms should be , not .

then I substituted and I got

where

since ..

after that I used the Maclaurin expansion of and I got

but the result should be

where's the mistake then?

thanks for any help