а) \(\sqrt{\frac{81\cdot25}{16}}=\frac{\sqrt{81}\cdot\sqrt{25}}{\sqrt{16}}=\frac{9\cdot5}{4}=\frac{45}{4}\);
б) \(\sqrt{\frac{9\cdot16}{25\cdot49}}=\frac{3\cdot4}{5\cdot7}=\frac{12}{35}\);
в) \(\sqrt{\frac{36}{49\cdot121}}=\frac{6}{7\cdot11}=\frac{6}{77}\);
г) \(\sqrt{\frac{121\cdot256}{25\cdot100}}=\frac{11\cdot16}{5\cdot10}=\frac{176}{50}=\frac{88}{25}\).
Ответ: а) \(\frac{45}{4}\); б) \(\frac{12}{35}\); в) \(\frac{6}{77}\); г) \(\frac{88}{25}\).