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1.

what is the avareage of series 1,5,9,---,81 ?

  • A. 39
  • B. 39
  • C. 39
  • D. 39

Answer: Option C

Explanation:

1+5+9.....+81  ধারাটির n তম পদ=a+n-1d⇒81=1+n-14⇒81=1+4n-4⇒81+3=4n⇒4n=84∴n=21∴ সমষ্টি =2122.1+21-14              =2122+80              =212×82              =21×41=861 সুতরাং গড়= 86121=41

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