Answers

2015-10-26T22:22:47+05:30
Sum=n/2(2a+(n-1)d)
513=n/2(2×54+(n-1)×-3)
1026=n(108-3n+3)
1026=n(111-3n)
1026=111n-3n^2
3n^2-111n+1026=0
n^2-37n+342=0
n^2-19n-18n+342=0
n(n-19)-18(n-19)=0
(n-19)(n-18)=0
n=18, n=19
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