The number abcd in the sequence abcd 9 dcba is 9. This is because the sequence is a palindrome, meaning it reads the same forwards and backwards. In palindromes, the middle digit is repeated, and in this case, the middle digit is 9. Therefore, the number abcd is 9.

This conclusion is based on the properties of palindromes, which are sequences that read the same forwards and backwards. In the given sequence abcd 9 dcba, the digits are arranged in a way that mirrors itself when read from left to right and right to left. The middle digit, which is 9, is the only digit that is repeated in the sequence, making it the number abcd.

According to the American Mathematical Association of Two-Year Colleges (AMATYC), palindromes are a fundamental concept in mathematics and are often used to teach students about number patterns and properties. The study of palindromes can help students develop critical thinking skills and an understanding of mathematical patterns. In this specific case, the palindrome property allows us to easily identify the number abcd as 9.