Answer:
x = -1/12
Explanation:
It is actually a rather simple proof. Before we get to that, it’s important to understand a couple of other things. Let’s consider the following infinite summation:
X = 1 - 1 + 1 - 1 + 1 - 1 ...
Rearranging the above equation a little bit, we get:
X = 1 - (1 - 1 + 1 - 1 + 1 - 1 ...)
If you look at the term inside the brackets, it in fact equals X. So let’s substitute that:
X = 1 - X
2X = 1
X = 1/2
Now let’s consider another sum.
Y = 1 - 2 + 3 - 4 + 5 - ...
Writing it in another way, we get:
Y = 0 + 1 - 2 + 3 - 4 + 5 + ...
Adding the above two equations:
Y + Y = (1 - 2 + 3 - 4 + 5 ...) + (0 + 1 - 2 + 3 - 4 + 5 ...)
Grouping the corresponding terms within the brackets, we get:
2Y = 1 + 0 - 2 + 1 + 3 - 2 - 4 + 3 + 5 - 4 ...
2Y = 1 - (2-1) + (3-2) - (4-3) + ...
2Y = 1 - 1 + 1 - 1 + 1 - 1 ...
But the summation on the right hand side is X. Let’s substitute it:
2Y = X
2Y = 1/2
Y = 1/4
Finally, let’s consider our original question at hand i.e. the sum of all natural numbers:
S = 1 + 2 + 3 + 4 + ...
We defined Y earlier as:
Y = 1 - 2 + 3 - 4 + ...
Subtracting Y from S:
S - Y = 1 - 1 + 2 + 2 + 3 - 3 + 4 + 4 + ...
S - Y = 4 + 8 + 12 + 16 + ...
We just calculated the value of Y to be 1/4. So let’s substitute it:
S - 1/4 = 4 x (1 + 2 + 3 + 4 + ...)
S - 1/4 = 4S
3S = -1/4
S = -1/12