Question 23
Suppose that there exist numbers $a,b,c$ and a function $f$ such that for any real numbers $x$ and $y$,
$$f(x+y)+f(x-y)=2f(x)+2f(y)+ax+by+c.$$
It is given that $f(2)=3$, $f(3)=-5$, and $f(5)=7$. Find the value of $f(123)$.
$$f(x+y)+f(x-y)=2f(x)+2f(y)+ax+by+c.$$
It is given that $f(2)=3$, $f(3)=-5$, and $f(5)=7$. Find the value of $f(123)$.