=2π*1/2∫r²e^(-r²)dr²=-π∫r²de^(-r²)=-πr²e^(-r²)+π∫e^(-r²)dr²=-πe^(-r²)=π
=2pi∫r^3e^(-r²)dr=pi∫r²e^(r²)dr²=pi∫xe^(-x)dx=piΓ(2)=pi
