Next consider a rigid body with most of its mass at 45 degrees to the y plane.
Ixx = sum (y^2)dm = 0.5+0.5 = 1
Iyy = sum(x^2) dm = 0.5+0.5 = 1
Ixy = sum(x*y)dm =0.5+0.5 = 1
det[M] = Ixx Iyy - Ixy Ixy = 0.25-0.25 =0
x/y = Ixy /(Ixx - )
x/y = 1/(1-0) = 1
normalising gives a unit vector at 45 degrees (along the line joining the masses).
x/y = 1/(1-2) = -1
normalising gives a unit vector at -45 degrees (at 90 degrees to the line joining the masses)
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