1. Define a good word as a sequence that only consits only of the letters A, B , C-some of these letters may not appear in sequence- and in which A is never immedialely followed by A, How many seven-letter good words are there? 2. Find the eighth term of a sequence 1440 , 1716 , 1848.., whose terms are formed by multiplying the corresponding terms of two arithmetic sequences. 3. Let N be the greatest integer multiple of 8 , no two of whose digits are the same. What is the reminder when N is divided by 1000?