Problem 3.1 At a point P, the stress tensor relative to axes Px1x2x3 has components tij. On the area element ds(1) having the unit normal ₁, the stress vector is t(1), and on area element ds(2) with normal 12 the stress vector is t(12). Show that the component of t(1) in the direction of 12 is equal to the component of t(2) in the direction of 1.