$a)\frac{{45}^o}{{360}^o}=$   $\frac{45}{360}=\frac{5}{40}=$   $\frac{1}{8}$  

$\frac{1}{8}\cdot \pi \cdot r^2=157$  

$\frac{1}{8}\cdot 3,14\cdot r^2=157\mid :3,14$   

$\frac{1}{8}\cdot r^2=50\mid \cdot 8$