What is the sum of all positive integers lying between 200 and 400 that are multiple of 7?
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A.P.=203,210,...........,399 An=A+(n-1) xd == 399=203+(n-1) x7 then we get, n=29 and we know that, Sn=n/2(A +An) ==29/2(203+399) Sn=8729
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