If a circular concentric hole is made in a disc, then about its own geometrical axis

If M be the mass of solid disc and m be the mass of remaining disc, Thenm=Mπb2 × π b2−a2= Mb2−a2b2Now for the solid disc  I=12  Mb2and for the hollow disc  I'=12m  a2+b2   = Mb2−a2 b2+a22b2                                             ∴   I'  <  IK = Radius of gyration of solid disc =  IM and K'= Radius of gyration of hollow disc =I'm∴   K  =  12Mb2M  =  b2 and K'=b2+a22  >  K