128+64+32+............. ধারাটির কোন পদ 1/2 ? 

A  9ম পদ  

B  7ম পদ  

C  8ম পদ  

D  10ম পদ 

Solution

Correct Answer: Option A

সূত্রানুসারে arn-1
  a=128
  r=64/128=1/2
সুতরাং ,
128×(1/2)n-1 =1/2
or,(1/2)n-1 =1/256
or,(1/2)n-1 =(1/2)8
or,n-1 =8
 or, n=8+1
  ∴  n=9

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