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If \(f(n)= 1 + 2 + 3 +\cdots+(n+1) \) and \(g(n)= \sum_{k=1}^{k=n} \dfrac{1}{f(k)}\), then the least value of \(n\) for which \(g(n)\) exceeds the value \(\dfrac{99}{100}\) is:

Question and options

If f(n)=1+2+3+⋯+(n+1)f(n)= 1 + 2 + 3 +\cdots+(n+1) and g(n)=∑k=1k=n1f(k)g(n)= \sum_{k=1}^{k=n} \dfrac{1}{f(k)}, then the least value of nn for which g(n)g(n) exceeds the value 99100\dfrac{99}{100} is:
  1. 198
  2. 99
  3. 200
  4. 199

Answer and explanation

Correct answer: D - 199

The least value of nn is 199. Since f(k)=(k+1)(k+2)2f(k)=\tfrac{(k+1)(k+2)}2, its reciprocal is 2k+1−2k+2\tfrac2{k+1}-\tfrac2{k+2}, and the sum telescopes to 1−2n+21-\tfrac2{n+2}. Exceeding 99100\tfrac{99}{100} requires n+2>200n+2>200, hence n>198n>198. At 198 the sum merely equals the threshold.

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