2. Solve the functional inequality $f\left(\frac{1-x}{1+x}\right) \geqslant x$.


2. Let $t=\frac{1-x}{1+x}$, then $x=\frac{1-t}{1+t}$. Substituting into the given functional inequality, we get $f(t) \geqslant \frac{1-t}{1+t}(t \neq-1)$. Therefore, $f(x) \geqslant \frac{1-x}{1+x}(x \neq-1)$.