Answer :

List of n natural number is


1, 2, 3, ……n


First term a = 1


Second term t1 = 2


Third term t3 = 3


Thus, common difference d = t3 – t2 = 3 – 2 = 1


Given Sn = 36


Thus, By using sum of nth term of an A.P. we will find it’s sum



Where, n = no. of terms


a = first term


d = common difference


Sn = sum of n terms


We need to find no. of terms n





n(1 + n) = 36 × 2 = 72


n2 + n – 72 = 0


n2 + 9n – 8n – 72 = 0


n(n + 9) – 8(n + 9) = 0


(n – 8)(n + 9) = 0


n – 8 = 0 or n + 9 = 0


n = 8 or n = – 9


Since, number of terms n can’t be negative


n = 8


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