Вводим полярные координаты
x= ρ cos θ
y= ρ sin θ
⇒
x^2+y^2=(ρ cos θ)^2 +(ρ sin θ)^2= ρ ^2(cos^2 θ +sin^2 θ )= ρ ^2*1= ρ ^2
x^2+y^2=9 ⇒
ρ ^2=9
ρ =3
0 ≤ ρ ≤ 3
π/2 ≤ θ ≤ π
dxdy= ρ d ρ d θ
∫ ∫ [blue]x[/blue]y[b]dxdy[/b]= ∫ ∫ [blue] ρ cos θ [/blue] ρ sin θ [b]ρ d ρ d θ [/b]= ∫_(0)^(3) ( ρ^3*∫_(π/2)^(π) cos θ sin θ d θ )d ρ =
= ∫_(0)^(3) [b]([/b] ρ^3(cos^2 θ /2)|_(π/2)^(π)[b])[/b] d ρ = ∫_(0)^(3) [b]([/b] ρ^3(1/2)(cos^2 π-cos^2(π/2)[b])[/b] d ρ =
=(1/2)*( ρ ^4/4)|_(0)^(3)=3^4/8=[b]81/8[/b]