3) (20 points) Modify the general (anisotropic) material stiffness matrix D for the case where the coefficients remain invariant under the transformation x x,y y and z -z, e.g. a reflection with respect to the (x, y) plane (the material has one plane of symmetry, (x,y) plane). (Hint: determine the direc- tion cosines for this transformation and then use the result of transformation of stresses and strains).