$a)l=2\pi r=2\pi \cdot 2=4\pi$

$b)l=2\pi \cdot 5=10\pi$

$c)l=2\pi \cdot 1,5=3\pi$

$d)l=2\pi \cdot 3,8=7,6\pi$

$e)l={\sout{2}}^1\pi \cdot \frac{1}{{\sout{2}}^1}=\pi$

$f)l=2\pi \cdot 2\frac{1}{5}=4\frac{2}{5}\pi$