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Rajan@2021
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Find the number of terms of the AP -12, -9, -6…….21. If 1 is added to each term of this AP then find the sum of all terms of the AP thus obtained.

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One of the basic and conceptual question from arithmetic progression chapter in which we have been asked to find the number of terms of the AP -12, -9, -6…….21. If 1 is added to each term of this AP then find the sum of all terms of the AP thus obtained.

Book – RS Aggarwal, Class 10, chapter 5C, question no 39.

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1 Answer

  1. l=a+(n−1)d

    where l= last term
    n=no. of terms
    a=first term
    21=−12+(n−1)3
    21+12+3=3n−
    36=3n
    n=12
    if 1 is added to each term then sequence become
    −11,−8,−5,.....22
    now
    sn​=n/2​[a+l]
    s12​=12/2​[−11+22]
    s12​=6[11]
    s12​=66

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