Quote by shizzle
So for example, lets say that i wish to calculate the MI for an axis perpendicular to plane of rectangle and passes through center of mass, this is how i'd go about it:
Using perpendicular axis theorem, Iz = Ix + Iy so
Iz = [m (a^2 + b^2)]/3
is this right?
What would happen then if the axis was still perpendicular to plane but passing through a corner?

No,it's wrong.Try to apply the definiton of the moment of inertia.Imagine an infinitesimal rectangle of sides 'dx' and 'dy' at a square distance of 'x^{2}+y^{2}' from the center.The center of the rectangle is the meeting point of the axis system.
Daniel.