The answer is actually $${\mathtt{\,-\,}}{\frac{{\mathtt{1}}}{{\mathtt{12}}}}$$
Let:
S=1+2+3+4+5+6+7+8+9+n
We know the answer to the following infinite sum:
S2=1-1+1-1+1-1+1-1+1-1+1-1+1-....=$${\frac{{\mathtt{1}}}{{\mathtt{2}}}}$$
We can therefore solve for this sum:
S3=1-2+3-4+5-6+7-8+9-....
We add them:
S3= 1-2+3-4+5-6+7-8...
S3= +1-2+3-4+5-6+7-8...
________________
2S3= 1-1+1-1+1-1+1-1+1-....
Therefore:
2S3=S2=$${\frac{{\mathtt{1}}}{{\mathtt{2}}}}$$
S3=$${\frac{{\mathtt{1}}}{{\mathtt{4}}}}$$
We can subtract S3 from S:
S-S3= 1+2+3+4+5+6+7+8+9-...
-[1- 2+3- 4+5- 6+7- 8+9-...
_________________________
4 + 8 + 12 + 16 +....
If we take a factor of 4 out:
S-S3=4(1+2+3+4+5+6+7+8+9+...)
The sum inside the parentheses is S:
S-S3=4(S)
We know S3=$${\frac{{\mathtt{1}}}{{\mathtt{4}}}}$$
S-$${\frac{{\mathtt{1}}}{{\mathtt{4}}}}$$=4(S)
Subtract S from each side:
$${\mathtt{\,-\,}}{\frac{{\mathtt{1}}}{{\mathtt{4}}}}$$=3S
Divide by 3:
$${\mathtt{\,-\,}}{\frac{{\mathtt{1}}}{{\mathtt{12}}}}$$=S
∞
∑ n+1=$${\mathtt{\,-\,}}{\frac{{\mathtt{1}}}{{\mathtt{12}}}}$$
n
-Daedalus