+865 If w, x, y, and z are real numbers satisfying:\(\begin{align*} w+x+y &= -2, \\ w+x+z &= 4, \\ w+y+z &= 19, \text{ and} \\ x+y+z &= 12, \end{align*}\)
what is wx + yz?
Matrix system:
\(\begin{pmatrix} 1 & 1 & 1 & 0 \\ 1 & 1 & 0 & 1 \\ 1 & 0 & 1 & 1 \\ 0 & 1 & 1 & 1 \end{pmatrix} \begin{pmatrix} w \\ x \\ y \\ z \end{pmatrix} = \begin{pmatrix} -2 \\ 4 \\ 19 \\ 12 \end{pmatrix} \)
Inverting the matrix leads to w = -1, x = -9, y = 6, z = 12, so wx + yz = 63.
