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, 2003

Number theory Difficulty 3.0 AMC 10/12 Prove it Canada

In the series of odd numbers $1+3+5-7-9-11+13+15+17-19-21-23-...$ the signs alternate every three terms, as shown. What is the sum of the first 300 terms of the series?

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Source: CEMC, University of Waterloo, licensed CC-BY-NC-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project. Solutions are the publisher's, linked not copied.