Ответ:

  1. \((\sqrt{3}\cdot\sqrt{7})^2-1=(\sqrt{21})^2-1=21-1=20\).
  2. \(\sqrt{9}+ (\sqrt{0.9})^2=3+0.9=3.9\).
  3. \(\sqrt{2.4}\cdot\sqrt{60}=\sqrt{144}=12\).
  4. \(\frac{3}{5}\sqrt{50}\cdot\sqrt{8}=\frac{3}{5}\sqrt{400}=\frac{3}{5}\cdot20=12\).

Ответ: 1) 20; 2) 3,9; 3) 12; 4) 12.