Problem

Source: Korea junior mathematical olympiad 2nd round 2019 december problem 6

Tags: algebra, functional equation, KJMO, function



Find all functions $f:\mathbb{R} \rightarrow \mathbb{R}$ which satisfies the followings. (Note that $\mathbb{R}$ stands for the set of all real numbers) (1) For each real numbers $x$, $y$, the equality $f(x+f(x)+xy) = 2f(x)+xf(y)$ holds. (2) For every real number $z$, there exists $x$ such that $f(x) = z$.