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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 2026-05-26

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