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NDA 2023 Maths · Mathematics · Sequences and Series · medium

In an arithmetic progression the 4th term is 11 and the 9th term is 31.

Single correct option

In an arithmetic progression the 44th term is 1111 and the 99th term is 3131. The sum of its first 2020 terms is
  1. A. 700700
  2. B. 720720
  3. C. 760760
  4. D. 740740
Show answer and solution

Answer: (D) 740740

Subtracting, 5d=205d=20, so d=4d=4 and a=11−3⋅4=−1a=11-3\cdot4=-1. Then S20=202 [2(−1)+19⋅4]=10⋅74=740S_{20}=\frac{20}{2}\,[2(-1)+19\cdot4]=10\cdot74=740. Using a=11a=11 or a=−4a=-4 in the formula gives the other tempting values.
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Original question written in the NDA 2023 Maths pattern and checked twice for the correct answer. Not an official question paper. How we write and check questions · Corrections policy · Report a mistake