Project Euler Problem 6

問題

The sum of the squares of the first ten natural numbers is,
1^(2) + 2^(2) + ... + 10^(2) = 385

The square of the sum of the first ten natural numbers is,
(1 + 2 + ... + 10)^(2) = 55^(2) = 3025

Hence the difference between the sum of the squares of the first ten natural numbers
and the square of the sum is 3025 − 385 = 2640.

Find the difference between the sum of the squares of the first one hundred natural
numbers and the square of the sum.

ソース

p (1..100).to_a.combination(2).inject(0) {|x, a|
	x + 2 * a[0] * a[1]
}

解答

25164150

感想

コンビネーション使えば余裕！
まぁこの解法を思いつくかがポイント