∫D∫sin√(x^2+y^2)dxdy=∫(0,2π)dθ∫(0,1)r^2dr=∫(0,2π)[1/3r^3](0,1)dθ=1/3∫(0,2π)[6π^3-0^3]dθ=2π^3[θ](0,2π)=2π^3(2π-0)=4π^4
