Doubt in the derivation of the relation between Young's modulus and Modulus of rigidity

Imagine a smaller (horizontal) square with its vertices at the four points where the force vectors contact the cube (at the center of the four faces). This is the shape of the square prior to loading. Now imagine that, when the forces are applied, the square deforms into a rhombus, with one of its diagonals increasing by the strain in the x direction and its other diagonal decreasing by an equal compressive strain in the y direction. The angle of the vertex decreases from 90 degrees to the new included angle of the rhombus. This change in angle, in radians, is the shear strain.