Ответ:

  1. \(\sqrt{0.25\cdot81}=\sqrt{20.25}=4.5\).
  2. \(\sqrt{16\cdot900}=\sqrt{14400}=120\).
  3. \(\sqrt{49\cdot121}=\sqrt{5929}=77\).
  4. \(\sqrt2\cdot\sqrt8=\sqrt{16}=4\).
  5. \(\sqrt5\cdot\sqrt{20}=\sqrt{100}=10\).