# Math Help - Function Question

1. ## Function Question

I'm currently on chapter 3 of Spivak's Calculus book, which is about functions. I'm confused about what this problem is asking:
Let $f(x)=1/(1+x)$ What is.. $f(x)+f(y)$
I don't understand how you can get an answer when f(y) isn't defined, or anything concerning y for that matter. I'm pretty sure that I'm just thinking about this the wrong way, and would appreciate some help.

2. ## Re: Function Question

If \displaystyle \begin{align*} f(x) = \frac{1}{1 + x} \end{align*} and you want to find \displaystyle \begin{align*} f(y) \end{align*}, substitute y as x.

3. ## Re: Function Question

So unless y or any other variable isn't previously defined, you simply treat it as a number?

Correct.