this may help you
The moment of inertia about an axis through the centre and perpendicular to the lamina is:
M(a^2 + b^2) / 12.
If h is the length of a semi-diagonal:
h^2 = (a/2)^2 + (b/2)^2
= (a^2 + b^2) / 4.
By the parallel axis theorem, the moment of inertial about an axis through the corner is:
M(a^2 + b^2) / 12 + Mh^2
= M(a^2 + b^2)(1/12 + 1/4)
= M(a^2 + b^2) / 3.
