Q1 of 85 Page 104

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

2, 6, 10, 14, ... up to 20 terms

Formula used.


Sn = [2a + (n–1)d]


d = an + 1–an


In the following A.P


a = 2,


d = a2–a1


= 6–2 = 4;


n = 20


Sn = [2a + (n–1)d]


= [2×2 + (20–1)4]


= 10[4 + 19×4]


= 10[4 + 76]


= 800


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