Question 40
Let $a$, $b$, $c$, $d$ be nonnegative real numbers for which $a^2 + b^2 = ac + bd$ holds and $c$, $d$ are not both zero. Find maximum and minimum value of the expression
$$ \frac{ad + bc - cd}{c^2 + d^2}. $$
$$ \frac{ad + bc - cd}{c^2 + d^2}. $$