Question 85
Let $D$ be the set of real numbers excluding $-1$. Find all functions $f: D \rightarrow D$ such that for all $x, y \in D$ satisfying $x \neq 0$ and $y \neq -x$, the equality
$$
(f(f(x)) + y) f\left(\frac{y}{x}\right) + f(f(y)) = x
$$
holds.
$$
(f(f(x)) + y) f\left(\frac{y}{x}\right) + f(f(y)) = x
$$
holds.