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 $\displaystyle f(x)=1/(1+x)$ What is.. $\displaystyle f(x)+f(y)$

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.