(a) Complete this proof of Theorem 1.3.1(b): Proof: Let U be any univ...

(a) Complete this proof of Theorem 1.3.1(b):

Proof: Let U be any universe.

The sentence is true in U

iff. . .

iffis true in U.

(b) Give a proof of part (b) of Theorem 1.3.1 that uses part (a).