Q2 of 85 Page 104

Find the sums indicated below:

3 + 6 + 9 + ... + 300

Formula used.


Sn = [2a + (n–1)d]


d = an + 1–an


In the sum above A.P follows


Is 3, 6, 9, ……, 300


In the following A.P


⇒ a = 3,


⇒ d = 6–3 = 3


As the last term is 300


an = a + (n–1)d


300 = 3 + (n–1)3


300 = 3 + 3n–3


3n = 300


⇒ n = = 100


Sn = [2a + (n–1)d]


= [2×3 + (100–1)3]


= 50[6 + 99×3]


= 50[6 + 297]


= 15150


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