20. Suppose that $a+x^{2}=2006, b+x^{2}=2007$ and $c+x^{2}=2008$ and $a b c=3$. Find the value of
$$
\frac{a}{b c}+\frac{b}{c a}+\frac{c}{a b}-\frac{1}{a}-\frac{1}{b}-\frac{1}{c} .
$$
20. Ans: 1 .
Rewrite the expression as
$$
\frac{a^{2}+b^{2}+c^{2}-b c-c a-a b}{a b c}=\frac{(a-b)^{2}+(b-c)^{2}+(c-a)^{2}}{2 a b c}
$$

Since $a-b=-1, b-c=-1$ and $c-a=2$, the result follows.