d
\({I_{nn'}}\) \(= M.I\) due to the point mass at \(B +\)

                     \(M.I\) due to the point mass at \(D +\)

                     \(M.I\) due to the point mass at \(C.\)

\(\begin{array}{l}
{I_{nn'}} = 2 \times m{\left( {\frac{\ell }{{\sqrt 2 }}} \right)^2} + m{\left( {\sqrt 2 \ell } \right)^2}\\
\,\,\,\,\,\,\,\, = m{\ell ^2} + 2m{\ell ^2} = 3m{\ell ^2}
\end{array}\)