13. a set s s r is said to be bounded from below if, and only if, there exists a real number c such that c s s for each s e s. for instance, each of the sets [0, ), (0, o), and q' are bounded from below because in each case every element of the set is greater than (or equal to) 0 determine whether the given set is bounded from below. expln a) z b) n d) (x2x+1> 0) f (+5> 0)