(b) aa, ab, ba
(c) Add b in front of a string, and add a or b at the end of a string.
(d) Let L be the language to be defined.
Basis Clause:
Inductive Clause: For any
, xa, xb and bx are all in L. Extremal Clause: Nothing is in L unless it is obtained from a finite number of applications of the Basis and Inductive Clauses.
and
for any string x, it holds for n = 0. 
holds for an arbitrary n. 
by the definition of 
by the property of REV. 
by the induction hypothesis. 
