$\begin{array}{l}
 {P_{ki}} = \frac{{\pi {r^2}\alpha }}{{{{360}^o}}} \\ 
 iz\,trugla: \\ 
 \cos \frac{\alpha }{2} = \frac{{r - h}}{r} \\ 
 \frac{\alpha }{2} = \arccos \frac{{r - h}}{r} \\ 
 \alpha  = 2\arccos \frac{{r - h}}{r} \\ 
 {P_{ki}} = \frac{{\pi {r^2}\alpha }}{{{{360}^o}}} = \frac{{\pi {r^2}}}{{{{360}^o}}}2\arccos \frac{{r - h}}{r} \\ 
 {P_{ki}} = \frac{{\pi {r^2}}}{{{{180}^o}}}\arccos \frac{{r - h}}{r} \\ 
 kad\,predjemo\,u\,radijane \\ 
 {P_{ki}} = {r^2}\arccos \frac{{r - h}}{r} \\ 
 \end{array}$