¿por qué este codigo devuelve el valor 6?

Question

What is the output of this code?
def func(x):
res = 0
for i in range(x):
res += i
return res
print(func(4))

Answer 1

Because it is a sum of 1 +2 + 3 + 4 + ... + n, then this sum is equal to:
n(n-1)/2
Replace n = 4 and the result is 6.
Cheers