Question

What is the sum of all positive integers lying between 200 and 400 that are multiple of 7?

Answer 1

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

Answer 2

Answer:

refers to the attachment