Issue #13108 has been reported by Eli Sadoff.

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Feature #13108: [Doc Request] Explicitly Range#sum
https://bugs.ruby-lang.org/issues/13108

* Author: Eli Sadoff
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
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While `Range#sum` exists because it implements `Enumerable#sum`, it is not noted how it works. While it does provide a hint that it is not an alias to `inject(:+)`, it is not explained that `(a..b).sum` implements `(b-a+1)*(a+b)/2` which is `O((log n)^2)` instead of `(a..b).inject(:+)` which is `O(n log n)`. It might be worth while either explicitly document `Range#sum` or at least documenting in `Enumerable#sum` that `Range#sum` has this behavior because the current documentation is not particularly descriptive and hides, what I think of, as quite a brilliant feature of 2.4.0. 



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