每日一题

Mathematics>Standard Multiple Choice

Read the following SAT test question and then click on a button to select your answer.

8, a, 14, b, 20, ...

The first term of the sequence above is 8. Which of the following could be the formula for finding the n th term of this sequence for any positive integer n?

(A) (2n) + 6
(B) (3n + 5
(C) (5n) + 3
(D) (6n) + 2
(E) (6n) + 5

答案和解析

答案：B

解析：

You do not know the value of the second term or the fourth term of the sequence, but you do know the values of the first, third, and fifth terms. The correct formula must give 8 as the value of the first term, 14 as the value of the third term, and 20 as the value of the fifth term. Choice E, (6n) + 5, gives 11, not 8, as the value of the first term. Choice A, (2n) + 6, gives 12, not 14, as the value of the third term. Choice C, (5n) + 3, gives 18, not 14, as the value of the third term. Choice D, (6n) + 2, gives 20, not 14, as the value of the third term. This leaves choice B, (3n + 5), which gives the correct values for the first, third, and fifth terms of the sequence.