(2)证明 设质量为m的刚体,质心为C, Jc=∑m2=∑m(x2+y2)=∑m2=∑m(x2+y x=x, y=yi+d ∑mx2+(y1+)2 y(2 ∑m2(x2+y2)+(∑m1)d2 +2a∑m2y ∑m,=m,∑my=m=0 y +md 刚体对通过质心轴的转动惯量具有最小值。⑵ 证明 设质量为m的刚体，质心为C， = = ( + ) 2 2 2 zC i i i i i J m r m x y = ' = ( ' + ' ) ' 2 2 2 z i i i i i J m r m x y  =  + + = = + [ ( ) ] ' , ' ' 2 2 J m x y d x x y y d z i i i  i i i i    + = + + i i i i i i d m y m x y m d 2 ( ) ( ) 2 2 2 2 0 J J md m m m y my z z C i i i C  = + = = =   , 刚体对通过质心轴的转动惯量具有最小值