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Question

Find the sum of the first 15 terms of the A.P. 5,8,11,14.....

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Solution

Given that: a = 5, n = 15, c.d = 8 - 5 = 3
Sn=n2[2a+(n1)d]
S15=152[2(5)+(151)3]=152[10+(14)(3)]=152×52
S15=390

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