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JAMB Mathematics 2006 Question 47

Question 47

Progressions

The sum of the first n positive integers is

  1. A.½n(n − 1)
  2. B.n(n + 1)
  3. C.n(n − 1)
  4. D.½n(n + 1)
Show the answer and explanation

Answer: D (½n(n + 1))

Explanation

1 + 2 + ... + n is an AP with first term 1 and last term n, so the sum is n/2 × (1 + n) = ½n(n + 1).