Add Gauss sum or sum of k naturals.

snippets/gauss-sum.md

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+---
+title: gauss-sum
+tags: math,beginner
+---
+
+Sums the numbers between 1 up to `limit` argument.
+
+- It is an implementation of the Gauss sum equation.
+
+```js
+const gaussSum = limit =>
+  (limit * (limit + 1)) / 2;
+```
+
+```js
+gaussSum(100); // 1 + 2 + 3 + 4 + ... + 100 = 5050
+```