P_k=\Pi r^2
r=0,2
P_k=\Pi *0,2^2\approx3,14*0,04\approx0,1256
…
r=\frac{2,2}{2}=1,1
P_k=\Pi *1,1^2\approx3,14*1,21\approx3,7994
…
Ob=2\Pi r
\frac{3\Pi}{4}=2\Pi r
3\Pi=8\Pi r
3=8 r
r=\frac{3}{8}
P_k=\Pi *(\frac{3}{8})^2\approx3,14*\frac{9}{64}\approx0,4415625