Question 79
Let $a$, $b$, $c$, $d$ be real numbers such that
$$
\min (20x+19, 19x+20) = (a x + b) - |c x + d|
$$
for all real numbers $x$. Find $a b + c d$.
$$
\min (20x+19, 19x+20) = (a x + b) - |c x + d|
$$
for all real numbers $x$. Find $a b + c d$.