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JEE Mains · Maths · STD 11 - 8. sequence and series

Let \(S_n=\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+\ldots\) upto \(n\) terms. If the sum of the first six terms of an A.P. with first term -p and common difference p is \(\sqrt{2026 \mathrm{~S}_{2025}}\), then the absolute difference betwen \(20^{\text {th }}\) and \(15^{\text {th }}\) terms of the A.P. is

  1. A \(20\)
  2. B \(90\)
  3. C \(45\)
  4. D \(25\)
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Answer & Solution

Correct Answer

(D) \(25\)

Step-by-step Solution

Detailed explanation

\begin{aligned} & \mathrm{Sn}=\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+\frac{1}{20} \ldots . \mathrm{N} \text { terms } \\ & \mathrm{S}_{2025}=\sum_{\mathrm{n}=1}^{2025} \frac{1}{\mathrm{n}(\mathrm{n}+1)}=\sum_{\mathrm{n}=1}^{2025}\left(\frac{1}{\mathrm{n}}-\frac{1}{\mathrm{n}+1}\rig…

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