Đáp án:
$\begin{array}{l}
a){2^3} - {\left( {\frac{1}{3}} \right)^0}.\sqrt {81} \\
= 8 - 1.9\\
= 8 - 9\\
= - 1\\
b)\left( {\frac{1}{2} - 1} \right).\left( {\frac{1}{3} - 1} \right).\left( {\frac{1}{4} - 1} \right)...\left( {\frac{1}{{998}} - 1} \right).\left( {\frac{1}{{999}} - 1} \right)\\
= \frac{{1 - 2}}{2}.\frac{{1 - 3}}{3}.\frac{{1 - 4}}{4}....\frac{{1 - 998}}{{998}}.\frac{{1 - 999}}{{999}}\\
= \frac{{ - 1}}{2}.\frac{{ - 2}}{3}.\frac{{ - 3}}{4}....\frac{{ - 997}}{{998}}.\frac{{ - 998}}{{999}}\left( {có:998\,dấu - } \right)\\
= \frac{1}{{999}}
\end{array}$