Q1 of 85 Page 104

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

—10, —12, —14, —16, ... up to 15 terms

Formula used.


Sn = [2a + (n–1)d]


d = an + 1–an


In the following A.P


a = –10,


d = a2–a1


= –12–(–10) = (–2);


n = 15


Sn = [2a + (n–1)d]


= [2×(–10) + (15–1)(–2)]


= [–20 + 14×(–2)]


= [–20–28]


= 15×(–24)


= –360


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