Find the `M.I.`

Explanation

`Mass//area = (m)/(pi(r_(1)^(2)-r_(2)^(2)))`
Taking a ring of radius `r` and thickness `dr`
`dA=2pidr`
`dm=(m)/(pi(r_(1)^(2)-r_(2)^(2)))xx2pirdr=(2mrdr)/((r_(1)^(2)-r_(2)^(2)))`
`I= intr^(2)dm=(2m)/((r_(1)^(2)-r_(2)^(2))) int_(r_(2))^(r_(1))r^(3) dr`
` =(2m)/((r_(1)^(2)-r_(2)^(2)))|(r_(4))/(4)|_(r_(2))^(r_(1))`
`= (1)/(2)(m(r_(1)^(4)-r_(2)^(4)))/((r_(1)^(2)-r_(2)^(2)))=(1)/(2)(m(r_(1)^(2)+r_(2)^(2))(r_(1)^(2)-r_(2)^(2)))/((r_(1)^(2)-r_(2)^(2)))`
`=(1)/(2)m(r_(1)^(2)+r_(2)^(2))`