Recursive formula find closed something

Hi!

There is this problem on recursive formulas I do not understand.

The problem is:

examine the recursive: $\displaystyle a_{n+1}=\frac{a_n}{1+a_n}$ where $\displaystyle a_0 =\frac{1}{2}$ and $\displaystyle n=0,1,2,3,4...$

Find a closed formula of the type $\displaystyle a_n = f(n)$ where $\displaystyle f(n) $is a expression that depends on n but not on for example $\displaystyle a_{n-1}$

I can probably do it if I would understand what they want, closed formula????

No idéa...where to start..