Sequence S is such that Sn=Sn−1+and S1=2Sequence A is such that An=An−1−1

题目

Sequence S is such that S_n=S_n−1+and S₁=2

Sequence A is such that A_n=A_n−1−1.5 and A₁=18.5

Quantity A

The sum of the terms in S from S₁ to S₁₃, inclusive

Quantity B

The sum of the terms from in A from A₁ to A₁₃, inclusive

选项

The quantity in Column A is greater.

The quantity in Column B is greater.

The two quantities are equal.

The relationship cannot be determined from the information given.

解析

数列 S 的前13项：2, 3.5, 5, 6.5, 8, 9.5, 11, 12.5, 14, 15.5, 17, 18.5, 20 数列 A 的前13项：18.5, 17, 15.5, 14, 12.5, 11, 9.5, 8, 6.5, 5, 3.5, 2, 0.5 可以看出，两个数列有12个相同项。如果将这12个相同项从两个数列中减去，可得：数列 S 的前13项：（划去12个相同项后，剩余）20 数列 A 的前13项：（划去12个相同项后，剩余）0.5 可以看出，数值A更大。答案：A

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