一道有限自动机的题目,求解,最好讲详细点,谢谢

Question

Find a regular expression corresponding to each of the following subsets of {a,b}*
a.The language of all string containing exactly two a's
b.The language of all string containing at least two a's
c.The language of all string that do not end with ab

Answer 1

(1) aa
(2) (a*b*)*aa(a*b*)*
(3) (a*b*)*bb + (a*b*)*ba + (a*b*)*aa

参考资料：sername