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JAMB Mathematics 1980 Question 21

Question 21

Progressions

Which of the formulae below represents the general terms of the following set of numbers {−1, 2/3, −1/2, 2/5, …} for n = 1, 2, 3, 4, …?

  1. A.2/(n + 1)
  2. B.(−1)n + 1 × 2/(n + 1)
  3. C.(−1)n × 2/(n + 1)
  4. D.n/(2n − 1)
  5. E.(−1)n × n/(2n + 1)
Show the answer and explanation

Answer: C ((−1)n × 2/(n + 1))

Explanation

The terms are 2/2, 2/3, 2/4, 2/5 with signs −, +, −, +. The sign is (−1)ⁿ, so the nth term is (−1)ⁿ × 2/(n + 1).