4. [4] The function $f$ satisfies
$$
f(x)+f(2 x+y)+5 x y=f(3 x-y)+2 x^{2}+1
$$
for all real numbers $x, y$. Determine the value of $f(10)$.
Answer: -49 Setting $x=10$ and $y=5$ gives $f(10)+f(25)+250=f(25)+200+1$, from which we get $f(10)=-49$.

Remark: By setting $y=\frac{x}{2}$, we see that the function is $f(x)=-\frac{1}{2} x^{2}+1$, and it can be checked that this function indeed satisfies the given equation.