Show in two dimensions that the maximum shear stress m = {[(xx yy)/2]2 + (xy)2}(1/2) is invariant

Show in two dimensions that the maximum shear stress τm = {[(σxx − σyy)/2]2 + (τxy)2}(1/2) is invariant under a rotation of the reference axes and is equal to one-half the difference of the major and minor principal stresses in the considered plane.