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1-js/06-advanced-functions/01-recursion/01-sum-to/task.md

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+importance: 5
+
+---
+
+# Sum all numbers till the given one
+
+Write a function `sumTo(n)` that calculates the sum of numbers `1 + 2 + ... + n`.
+
+For instance:
+
+```js no-beautify
+sumTo(1) = 1
+sumTo(2) = 2 + 1 = 3
+sumTo(3) = 3 + 2 + 1 = 6
+sumTo(4) = 4 + 3 + 2 + 1 = 10
+...
+sumTo(100) = 100 + 99 + ... + 2 + 1 = 5050
+```
+
+Make 3 solution variants:
+
+1. Using a for loop.
+2. Using a recursion, cause `sumTo(n) = n + sumTo(n-1)` for `n > 1`.
+3. Using the [arithmetic progression](https://en.wikipedia.org/wiki/Arithmetic_progression) formula.
+
+An example of the result:
+
+```js
+function sumTo(n) { /*... your code ... */ }
+
+alert( sumTo(100) ); // 5050
+```
+
+P.S. Which solution variant is the fastest? The slowest? Why?
+
+P.P.S. Can we use recursion to count `sumTo(100000)`?