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Deepak Bora
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How many terms of the AP 21,18,15,…..must be added to get the sum 0?

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This is the Important question based on Arithmetic progression Chapter of R.S Aggarwal book for ICSE & CBSE Board.
Here you have to find the number of terms which must be added to get the given sum.
Question Number 19 of Exercise 11 C of RS Aggarwal Solution

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1 Answer

  1. Given A.P :
    21,18,15,…
    First term (a)=21,
    Common difference (d)
    =a_{2}-a_{1}
    =18-21
    =-3
    Let Sum of n terms = 0
    S_{n}=0
    n_2[2a+(n-1)d]=0
    2a+(n-1)d=0
    2×21+(n-1)(-3)=0
    42-3n+3=0
    45-3n=0
    n=-45/-3
    n=15
    Therefore,
    Sum of 15 term is 0

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