$\text{a)}\sqrt{4\cdot 81}=\sqrt{4}\cdot \sqrt{81}=2\cdot 9=18$

$\sqrt{25\cdot 0,36}=\sqrt{25}\cdot \sqrt{0,36}=5\cdot 0,6=3$

$\sqrt{0,09\cdot 361}=\sqrt{0,09}\cdot \sqrt{361}=0,3\cdot 19=5,7$

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$\text{b)}\sqrt{\frac{121}{144}}=\frac{\sqrt{121}}{\sqrt{144}}=\frac{11}{12}$

$\sqrt{\frac{361}{400}}=\frac{\sqrt{361}}{\sqrt{400}}=\frac{19}{20}$

$\sqrt{\frac{576}{625}}=\frac{\sqrt{576}}{\sqrt{625}}=\frac{24}{25}$

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$\text{c)}\sqrt{2}\cdot \sqrt{8}=\sqrt{2\cdot 8}=\sqrt{16}=4$

$\sqrt{6}\cdot \sqrt{1,5}=\sqrt{6\cdot 1,5}=\sqrt{9}=3$

$\sqrt{2}\cdot \sqrt{5}\cdot \sqrt{10}=\sqrt{2\cdot 5}\cdot \sqrt{10}=\sqrt{10}\cdot \sqrt{10}=\sqrt{100}=10$

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$\text{d)}\frac{\sqrt{54}}{\sqrt{6}}=\sqrt{\frac{54}{6}}=\sqrt{9}=3$

$\frac{\sqrt{3}}{\sqrt{12}}=\sqrt{\frac{3}{12}}=\sqrt{\frac{1}{4}}=\frac{1}{2}$

$\frac{\sqrt{10}\cdot \sqrt{14}}{\sqrt{35}}=\frac{\sqrt{10\cdot 14}}{\sqrt{35}}=\frac{\sqrt{140}}{\sqrt{35}}=\sqrt{\frac{140}{35}}=\sqrt{4}=2$