Ответ:

  1. \(5^6\cdot5^{21}=5^{6+21}=5^{27}\).
  2. \(3^{-20}\cdot3^{15}=3^{-20+15}=3^{-5}=\frac{1}{3^5}=\frac{1}{243}\).
  3. \(8^{-2}\cdot8^{-1}=8^{-2-1}=8^{-3}=\frac{1}{8^3}=\frac{1}{512}\).

Ответ: 1) \(5^{27}\); 2) \(\frac{1}{243}\); 3) \(\frac{1}{512}\).