Q1 of 85 Page 104

Find the sum of the first n terms of the following A.P. as asked for:

up to 18 terms

Formula used.


Sn = [2a + (n–1)d]


d = an + 1–an


In the following A.P


a = ,


d = a2–a1


= = 1;


n = 18


Sn = [2a + (n–1)d]


= [2× + (18–1)1]


= 9[ + 17]


= 9[]


= 159


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