the principal values of a stress tensor t are t1 ¼ 10 mpa; t2 ¼ 10 mpa, and t3 ¼ 30 mpa. if the matrix of the stress is given by ½t ¼ t11 0 0 012 0 2 t33 2 4 3 5 10 mpa; find the values of t11 and t33.