2. 外语
  3. The numbers 3, 10, 17, 24, 31, and 38 are the first six terms of an infinite sequence in which each term after the first is 7 gr

The numbers 3, 10, 17, 24, 31, and 38 are the first six terms of an infinite sequence in which each term after the first is 7 gr

admin2009-05-19  46
问题 The numbers 3, 10, 17, 24, 31, and 38 are the first six terms of an infinite sequence in which each term after the first is 7 greater than the preceding term. What is the 45th term of the sequence?
选项 A、308
B、311
C、312
D、315
E、318
答案B
解析 数字3，10，17，24，31和38是一个无穷数列(infinite sequence)的前六项，该数列的每一项都比前一项大7，问这个数列的第45项是多少?
解：本题的正确答案是(B)。由题意可知，该数列是一个等差数列，其首项a1=3，其公差为d=7，根据等差数列的通项公式可得：a45=a1+(n-1)d=3+(45-1)×7=311
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