Iѯη = ∫A (ѯη)dA = ∫A (y*cosα-x*sinα)(x*cosα+y*sinα)dA = cos²α ∫A (xy) dA + sinαcosα ∫A (y²) dA - sinαcosα ∫A (x²) dA - sin²α ∫A (xy) dA = Ixy * cos2α + sinαcosα * (Ix - Iy) = ((Ix - Iy)/2) * sin2α + Ixy * cos2α
Put that in your pipe and smoke it