At a point in an elastic material there are two mutually perpendicular planes, one of which carries a direct tensile stress at 50N/mm2 and a shear stress of 40N/mm2, while the other plane is subjected to a direct compressive stress of 35 N/mm2 and a complementary shear stress of 40 N/mm2. Determine the principal stresses at the point, the position of the planes on which they act and the position of the planes on which there is no normal stress.