Discuss Forum

1. how many terms of arithmetic progression (A.P) 21,18,15,12...must be taken to give the sum zero

  • A. 10
  • B. 10
  • C. 10
  • D. 10

Answer: Option B

Explanation:

আমরা জানি, n তম পদের সমষ্টি Sn= n/2{2a+(n-1)d}

এখানে, a=21, d= 18-21= -3, Sn= 0, n=?

প্রশ্নমতে, 0= n/2{(2×21)+(n-1)-3}

বা, n/2(42-3n+3)=0

বা, 45-3n= 0×2/n

বা, 45-3n= 0

বা, 3n=45

বা, n=15


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