I have to express x as a function of y in the equation:
y=x/(1-x)
The back of the book says that the answer is y/(1+y). I'm really stuck on how they got there.
I'll explain what he did^^^ Basically "x is a function of y" means the value of x depends on the value of y.
The equation y=x/(1-x) is solved for y, and you can't determine what the value of x would be if you were to know y. So all you do is solve for x by moving everything around as shown in the post above.
properly it would be $\displaystyle f(y)=y/(y+1)$