Question 29
Real numbers $a, b, c, d$ are given. Solve the system of equations (unknowns $x, y, z, u$)
$$
\begin{cases}
x^2 - yz - zu - yu = a \\
y^2 - zu - ux - xz = b \\
z^2 - ux - xy - yu = c \\
u^2 - xy - yz - zx = d
\end{cases}
$$
$$
\begin{cases}
x^2 - yz - zu - yu = a \\
y^2 - zu - ux - xz = b \\
z^2 - ux - xy - yu = c \\
u^2 - xy - yz - zx = d
\end{cases}
$$