(p →q)↔(¬q → ¬p) and show that it is a tautology. Writing p → q as U
and ¬q → ¬p as V, the details are displayed in Table 1.12. Since the last column
of Table 1.12 is a tautology, U and V are logically equivalent. Notice that the
columns U → V and V → U are tautologies to make this last column a tautology.