Olympiad Maths Prep

Track / Stage 3 / 11 of 260 #11 of 2000

Problem 11

AMC 10/12, early questions
Algebra Difficulty 3.0 Find the answer

Given that the sum of the first nn terms of an arithmetic sequence is SnS_n, and S1006>S1008>S1007S_{1\,006} > S_{1\,008} > S_{1\,007}, find the positive integer value(s) of nn that satisfy SnSn1<0S_n S_{n-1} < 0.

A: 20152\,015
B: 20132\,013
C: 20142\,014
D: 20162\,016

Official solution

[Analysis]

According to the properties of the arithmetic sequence and the formula for the sum of the first nn terms, we have (S1008S1007)>0(S_{1\,008} - S_{1\,007}) > 0, which implies a1008>0a_{1\,008} > 0. From S1006>S1008S_{1\,006} > S_{1\,008}, we get (S1008S1006)0(S_{1\,008} - S_{1\,006}) 0 and S2014=2014(a1+a2014)2=2014(a1007+a1008)20S_{2\,014} = \frac{2\,014(a_1 + a_{2\,014})}{2} = \frac{2\,014(a_{1\,007} + a_{1\,008})}{2} 0, which implies a1008>0a_{1\,008} > 0.

From S1006>S1008S_{1\,006} > S_{1\,008}, we get (S1008S1006)0(S_{1\,008} - S_{1\,006}) 0 and S2014=2014(a1+a2014)2=2014(a1007+a1008)2<0S_{2\,014} = \frac{2\,014(a_1 + a_{2\,014})}{2} = \frac{2\,014(a_{1\,007} + a_{1\,008})}{2} < 0.

Therefore, the positive integer value of nn that satisfies SnSn1<0\boxed{S_n S_{n-1} < 0} is n=2015n = \boxed{2\,015}.

Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic, difficulty and ordering added by this site.