Question 96
Find all quadruples of real numbers $(a, b, c, d)$ such that
$$
a = b c + c d, \quad b = c d + d a, \quad c = d a + a b \text{ and } d = a b + b c
$$
$$
a = b c + c d, \quad b = c d + d a, \quad c = d a + a b \text{ and } d = a b + b c
$$