18. There is a sequence, the first number is 6, the second number is 3, starting from the second number, each number is 5 less than the sum of the number before it and the number after it. What is the sum of the first 200 numbers in this sequence, from the first number to the 200th number?
【Answer】 999
【Solution】The first number is 6, the second number is 3. According to the rule "from the second number, each number is 5 less than the sum of the number before it and the number after it," the third number is $3+5-6=2$; similarly, the subsequent numbers are: $6, 3, 2, 4, 7, 8, 6, 3, 2, 4 \ldots \ldots$. It is easy to see that every 6 numbers form a cycle. $200 \div 6=33 \ldots \ldots 2$. Therefore, the sum of the first 200 numbers is $33 \times(6+3+2+4+7+8)+6+3=999$.