If $f(x)=log\bigg(\large\frac{1+x}{1-x}\bigg)$, then $f(x)+f(y)$ = ?

Question

$\begin{array}{1 1} f(x+y) \\f(xy) \\f\bigg(\large\frac{x+y}{1+xy}\bigg) \\ f\bigg(\large\frac{x+y}{1-xy}\bigg) \end{array}$