J=∫∫(R*sina)^2*(m/(pi*R^2))dR*Rda (a从0到2pi,R从0到r)=∫∫(m/pi)R*(sina)^2 dRda=∫(m/(2pi))r^2*(1/2)(1-cos2a)da=(m/4pi)r^2*2pi=(1/2)mr^2
