Sum of first n numbers and 2-element subsets
I know it's not strictly a programming question but computer scientists might know the answer. why is the sum of the first n numbers equal to the number of 2-element subsets? 
Sum of first n numbers and 2-element subsets
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I know it's not strictly a programming question but computer scientists might know the answer. why is the sum of the first n numbers equal to the number of 2-element subsets? 