Question 80
Find all functions $f: \mathbb{R} \rightarrow \mathbb{R}$ such that
$$
f(a-b) f(c-d)+f(a-d) f(b-c) \leq (a-c) f(b-d)
$$
for all real numbers $a, b, c$ and $d$.
$$
f(a-b) f(c-d)+f(a-d) f(b-c) \leq (a-c) f(b-d)
$$
for all real numbers $a, b, c$ and $d$.