number of nine-character palindromes that can be formed using the letters of the alphabet such that no letter appears more than twice in each of them. 1.8. Find the number of ways to form a four-letter sequence using the letters A, B, C, D, and E if (a) repetitions of letters are permitted, (b) repetitions are not per mitted, (c) the sequence contains the letter A but repetitions are not permitted, and (d) the sequence contains the letter A but repetitions are permitted. E 133 OF 445 > 30% read Book Air