20. Calcular el valor de $x$ al resolver el sistema de ecuaciones:
$$ \begin{cases} x + y + z = 0 & (1) \\ (b + c)x + (a + c)y + (a + b)z = 0 & (2) \\ bcx + acy + abz = 1 & (3) \end{cases} $$

$$ \begin{array}{lll} \text{(a) } \frac{1}{(a - b)(a - c)} & \text{(b) } \frac{1}{(b - a)(b - c)} & \text{(c) } \frac{1}{(a - c)(b - c)} \\ \text{(d) } \frac{a}{(b - a)(b - c)} & \text{(e) } \frac{b}{(b - a)(b - c)} & \end{array} $$