\[{\text{Вычислите: }}\frac{1}{{\frac{1}{{18}} - \frac{1}{{21}}}}\]
\[\begin{array}{l}1){\text{ }}\frac{1}{{18}} - \frac{1}{{21}} = \frac{1}{{3 \cdot 6}} - \frac{1}{{3 \cdot 7}} = \\\frac{7}{{3 \cdot 6 \cdot 7}} - \frac{6}{{3 \cdot 6 \cdot 7}} = \frac{1}{{3 \cdot 6 \cdot 7}} = \frac{1}{{126}}\\2){\text{ }}\frac{1}{{\frac{A}{B}}} = \frac{B}{A}\\\frac{1}{{\frac{1}{{18}} - \frac{1}{{21}}}} = \frac{1}{{\frac{1}{{126}}}} = \frac{{126}}{1} = 126\end{array}\]
\[126\]
