The letters AA appear five times in a row, AB, BA and BB appear three times each.
Solution:
Starting from the first item, suppose that each paragraph consisting of a is A 1, A2, A3, and
The paragraphs composed of b are B 1, B2, b3, ...
The arrangement shapes that meet the conditions can only be such combinations.
A 1 B 1 A2 B2 A3 B3 a4 ①
Or b1a1b2a2b3b3b4 ②.
Each segment contains at least 1 A (or B), occupying 7 bits and requiring 8 bits.
How to allocate the remaining five A's and three B's to form five AA's and three BB's?
If a paragraph consists of (5+ 1) A, then only five AA can be formed in this paragraph;
The remaining five A's and three B's can only be allocated in this way.
For ①, put A with C (8,5) = 56 (species), put B with C (5,3) =10 (species), * * * 56× 10 = 560 (species).
For ②, put A with c (7,5) = 21(species), put B with c (6,3) = 20 (species), * * * 2 1× 20 = 420 (species).
Therefore, the arrangement that meets the conditions is always * * * 560+420 = 980 (species).
This topic is too difficult.