 In the first case, "z" = "a"("y" + "x"), therefore "y"− "("−"x)" = "z"/"a"  is in  "K"  with  −"x"  in  "F" .Hence

therefore "ψ"("z") ≥ 0.In the second case, "z" = "a"("x" − "y"), therefore "y" = "x" − "z"/"a".