 It is always possible to rotate the coordinates in such a way that in the new system there is no "x'y'" term.Substituting equations () and () into equation (), we obtain

where

If formula_1 is selected so that formula_22 we will have formula_23 and the "x'y'" term in equation () will vanish.When a problem arises with "B", "D" and "E" all different from zero, they can be eliminated by performing in succession a rotation (eliminating "B") and a translation (eliminating the "D" and "E" terms).