$a){\left(\frac{3}{7}\right)}^2=\frac{3}{7}\cdot \frac{3}{7}=\frac{9}{49}$

${\left(\frac{2}{5}\right)}^2=\frac{2}{5}\cdot \frac{2}{5}=\frac{4}{25}$

${\left(\frac{8}{7}\right)}^2=\frac{8}{7}\cdot \frac{8}{7}=\frac{64}{49}=1\frac{15}{49}$

${\left(1\frac{1}{8}\right)}^2={\left(\frac{9}{8}\right)}^2=\frac{9}{8}\cdot \frac{9}{8}=\frac{81}{64}=1\frac{17}{64}$

 

$b){\left(\frac{1}{2}\right)}^3=\frac{1}{2}\cdot \frac{1}{2}\cdot \frac{1}{2}=\frac{1}{8}$

${\left(\frac{2}{3}\right)}^3=\frac{2}{3}\cdot \frac{2}{3}\cdot \frac{2}{3}=\frac{4}{9}\cdot \frac{2}{3}=\frac{8}{27}$

  ${\left(2\frac{1}{2}\right)}^3={\left(\frac{5}{2}\right)}^3=\frac{5}{2}\cdot \frac{5}{2}\cdot \frac{5}{2}=\frac{25}{4}\cdot \frac{5}{2}=\frac{125}{8}=15\frac{5}{8}$  

${\left(1\frac{1}{4}\right)}^3={\left(\frac{5}{4}\right)}^3=\frac{5}{4}\cdot \frac{5}{4}\cdot \frac{5}{4}=\frac{25}{16}\cdot \frac{5}{4}=\frac{125}{64}=1\frac{61}{64}$