Ответ:

  1. \(\frac{4}{\sqrt{15}}\cdot\frac{\sqrt{15}}{\sqrt{15}}=\frac{4\sqrt{15}}{15}\).
  2. \(\frac{8}{\sqrt2}\cdot\frac{\sqrt2}{\sqrt2}=\frac{8\sqrt2}{2}=4\sqrt2\).
  3. \(\frac{42}{5\sqrt7}\cdot\frac{\sqrt7}{\sqrt7}=\frac{42\sqrt7}{35}=\frac{6\sqrt7}{5}\).