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JAMB Mathematics 1994 Question 21

Question 21

Progressions

Find the nth term of the sequence 3, 6, 10, 15, 21, ...

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

Answer: C ((n + 1)(n + 2)/2)

Explanation

Test n = 1 and n = 2: (n + 1)(n + 2)/2 gives 2×3/2 = 3 and 3×4/2 = 6, matching the sequence. The other formulas fail.