If the small strain tensor at a point in a body made of a moldable plastic could be determined experimentally by measuring the unit extensions in six directions at the point (the coordinate directions and the three directions bisecting the angles between the axes: em for the direction between X1 and X2, en between X2 and X3, et between X3 and X1), determine the shear strain components E12, E23, and E31 in terms of the six measured unit extensions.