$a)\sqrt{81}=\underline{\underline{9}},\text{bo}{9}^2=9\cdot 9=81$

$b)\sqrt{0}=\underline{\underline{0}},\text{bo}{0}^2=0\cdot 0=0$

$c)\sqrt{0,04}=\underline{\underline{0,2}},\text{bo}{0,2}^2=0,2\cdot 0,2=0,04$

$d)\sqrt{0,64}=\underline{\underline{0,8}},\text{bo}{0,8}^2=0,8\cdot 0,8=0,64$

$e)\sqrt{1,21}=\underline{\underline{1,1}},\text{bo}{1,1}^2=1,1\cdot 1,1=1,21$

$f)\sqrt{\frac{25}{4}}=\frac{5}{2}=\underline{\underline{2\frac{1}{2}}},\text{bo}{\left(\frac{5}{2}\right)}^2=\frac{5}{2}\cdot \frac{5}{2}=\frac{25}{4}$

$g)\sqrt{\frac{16}{49}}=\underline{\underline{\frac{4}{7}}},\text{bo}{\left(\frac{4}{7}\right)}^2=\frac{4}{7}\cdot \frac{4}{7}=\frac{16}{49}$

$h)\sqrt{2\frac{7}{9}}=\sqrt{\frac{25}{9}}=\frac{5}{3}=\underline{\underline{1\frac{2}{3}}},\text{bo}{\left(\frac{5}{3}\right)}^2=\frac{5}{3}\cdot \frac{5}{3}=\frac{25}{9}=2\frac{7}{9}$

  $i)\sqrt{7\frac{1}{9}}=\sqrt{\frac{64}{9}}=\frac{8}{3}=\underline{\underline{2\frac{2}{3}}},\text{bo}{\left(\frac{8}{3}\right)}^2=\frac{64}{9}=7\frac{1}{9}$  

$j)\sqrt{3\frac{1}{16}}=\sqrt{\frac{49}{16}}=\frac{7}{4}=\underline{\underline{1\frac{3}{4}}},\text{bo}{\left(\frac{7}{4}\right)}^2=\frac{49}{16}=3\frac{1}{16}$