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