Deepak Bora
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The sum of the first n terms of an AP is given by S_n=(2n^2+5n). Find the nth terms of the AP.

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In this Question the sum of first n terms of the AP is given. You have to find its nth term.
This is the Important question based on Arithmetic progression Chapter of R.S Aggarwal book for ICSE & CBSE Board.
This is the Question Number 24 Of Exercise 11 C of RS Aggarwal Solution


1 Answer

  1. Sum of first n terms of AP
    S_n=2n^2+5n————-1st equation
    by putting the value of n in the 1st equation
    So,the First term is
    now putting the value of n=2 to get the sum of two terms
    This means first term + second term = 18
    but first term =7
    as calculated above so,
    second term = 18-7 = 11
    common difference becomes,
    11-7 = 4
    So the AP becomes, 7, 11, 15, ….nth term
    = a + (n – 1)d
    = 7 + (n-1)4
    = 7 + 4n – 4
    = 3 + 4n

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