Translate the following inference to logic notation: For all x, if x_6 takes a plane, then x_6 is sad. For all x, if x_6 were sad, then x_6 is taking a plane. For all x, if x_6 were sad, then x_6 is taking a plane. Therefore for all x, if x_6 were sad, then x_6 is taking a plane.
forall x: P(x) -> Q(x). forall x: Q(x) -> P(x). forall x: Q(x) -> P(x). Therefore forall x: Q(x) -> P(x).