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1. 1 + 5 + 9+ ------81 = ?

  • A. 21
  • B. 21
  • C. 21
  • D. 21

Answer: Option B

Explanation:

  1   +   5   +   9   +   .......   +  81  ধারাটির , n - তম পদ   =  a   +   (n - a)d বা, 81  =  1   +   (n - 1) 4 বা, 81  = 1    +  n  - 4  বা 4n  = 84  ∴ n = 21  ∴ সমষ্টি  = 2122.1+21-14=2122+80=21×41=861

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