$a)P=\pi \cdot {\left(8cm\right)}^2=64\pi c{m}^2$

$b)P=\pi \cdot {\left(21cm\right)}^2=441\pi c{m}^2$

$c)P=\pi \cdot {\left(1,6cm\right)}^2=2,56\pi c{m}^2$

$d)P=\pi \cdot {\left(\frac{3}{4}cm\right)}^2=\frac{9}{16}\pi c{m}^2$

$e)P=\pi \cdot {\left(0,02cm\right)}^2=0,0004\pi c{m}^2$