Question 142
Find all injective functions $f: \mathbb{R} \rightarrow \mathbb{R}$ such that all real numbers have $x \neq y$
$$
f\left(\frac{x+y}{x-y}\right)=\frac{f(x)+f(y)}{f(x)-f(y)}
$$
$$
f\left(\frac{x+y}{x-y}\right)=\frac{f(x)+f(y)}{f(x)-f(y)}
$$