Why is sum so much faster than inject(:+)?

Short answer For an integer range : Enumerable#sum returns (range.max-range.min+1)*(range.max+range.min)/2 Enumerable#inject(:+) iterates over every element. Theory The sum of integers between 1 and n is called a triangular number, and is equal to n*(n+1)/2. The sum of integers between n and m is the triangular number of m minus the triangular number of n-1, which is equal to m*(m+1)/2-n*(n-1)/2,

[tuto0621]

sumは (range.max-range.min+1)*(range.max+range.min)/2 なので高速