The most convenient method is the replacement method.

Let t = 1/x (t ≠ 0) then x =1/t.

So f (t) =1/(1/t+1) is simplified to x/(x+ 1).

F (x) = x/(x+ 1) when t is replaced by x.

Here we go. I changed X to T, which means that the range of X is x≠0.

The reason why-1 is not used is because, like upstairs, it is the default. Moreover, this is the characteristic of substitution method. After substitution, the functions t and x are consistent, because the brackets are a whole and equal.

You got it? Don't ask questions ~ ~