#### Exercise

For each natural number n, we set: S_{n} = 1 + 2 + 3 + … + n. Write a Python algorithm that finds the integer n for a given Sn. Example if S_{n} = 10 then n = 4 since 1 + 2 + 3 + 4 = 10.

#### Solution

`def backSum(Sn):`

# Initializing value of n

n = 1

i = 0

while ( n < Sn):

i = i + 1

n = n + i

return i

# Testing algorithm for Sn = 10 = 1+2+3+4. The algorothm must be return 4

print(backSum(10))

**Younes Derfoufi**

my-courses.net

my-courses.net