Find L_4. Show that L_k is finite. Let A = {a, b, c}. Describe the language L_r for each regular expression: r = ab*c* r = a* b*c* Let A = {a, b}. Find a regular expression r such that the language L_r consists of all words w where: w begins with a^2 and ends with b^2 w contains an even number of a's Consider the grammar S rightarrow aS |aSbS| epsilon This grammar is ambiguous. Show in particular that the string aab has two: Parse trees Leftmost derivations Rightmost derivations Find an unambiguous grammar for this language