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Alternating Signs in Sequences Using Powers of (−1)(-1)

The expressions (−1)n(-1)^n and (−1)n+1(-1)^{n+1} are used in sequence formulas to make the terms alternate between positive and negative values. Because (−1)(-1) raised to an odd power equals −1-1 and raised to an even power equals 11, multiplying a sequence's general term by one of these factors flips the sign of every other term.

  • (−1)n(-1)^n produces the pattern −1,1,−1,1,−1,…-1, 1, -1, 1, -1, \dots (the first term is negative).
  • (−1)n+1(-1)^{n+1} produces the pattern 1,−1,1,−1,1,…1, -1, 1, -1, 1, \dots (the first term is positive).

Which expression to use depends on whether the first term of the sequence should be positive or negative.

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Updated 2026-06-18

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Intermediate Algebra @ OpenStax

Ch.12 Sequences, Series and Binomial Theorem - Intermediate Algebra @ OpenStax

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