Question

if the sum of first 9 terms of an A.P. .nets 81 and that of first 17 terms is 289 ,find the sum of first n terms.

Answer 1

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Answer 2

9/2(2a+(9-1)d=81----1

17/2(2a+(17-1)d=289----2

By solving these two equations we will get value of a and d

a= 1and d=2 then sum of n terms will be

n/2(2×1+(n-1)2

n^2