Axes fixed by directions given by Eqs.(7a) and (7b) are called the principal axes of inertia of a thin and rigid plate. These axes are mulually perpendicular because, taking the case.... as an example, the product of corresponding slopes..... and .... is equal to... Rotation of the plate around the axis fixed with directions () for ...... and ..... for..... results in the minimum kinetic energy and solves the problem that we initially posed.