26. \(\frac{1}{\frac{1}{18}+\frac{1}{21}}\)

\(\frac{1}{18}+\frac{1}{21}=\frac{7}{126}+\frac{6}{126}=\frac{13}{126}\).

\(1:\frac{13}{126}=\frac{126}{13}=9\frac{9}{13}\).

Ответ: \(\frac{126}{13}\).