y=sqrt(R^2-x^2)- уравнение окружности в верхней полуплоскости
в первой четверти
0 ≤ x ≤ R
dl=sqrt(1+(y`)^2)dx
y`=(1/2sqrt(R^2-x^2))*(R^2-x^2)`=-2x/(2sqrt(R^2-x^2))=-x/sqrt(R^2-x^2)
1+(y`)^2=1+(x^2)/(r^2-x^2)=R^2/(R^2-x^2)
sqrt(1+(y`)^2)=R/sqrt(R^2-x^2)
∫ _(L)x^2*ydl= ∫ ^(R)_(0)x^2*sqrt(R^2-x^2)* (R/sqrt(R^2-x^2)dx=
= R∫ ^(R)_(0)x^2dx=r*(x^3/3)|^(R)_(0)=[b]R^4/3[/b]