Sir please give me a detailed solution of this question in which we have to find the number of terms must be added to to the given series so that its sum must be 0.
Book – RS Aggarwal, Class 10, chapter 5C, question no 8
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Here,
a1=21
d=−3
Let n terms be added to get the sum 0.
n/2[2a+(n−1)d]=0
2a+(n−1)d=0
42+(n−1)(−3)=0
42−3n+3=0
3n=45
n=15
Hence, 15 terms of the AP must be added to get the sum 0.