Only one of the following relations below is true for all
integers~$n\ge1$.  Which one is it?
\correct $7^n-6n-1$ is divisible by $36$
\wrong $1+2+\cdots+n=\frac{n(n-1)}2$
\wrong $\sum_{k=1}^n(0.99999)^k\le100$
\wrong $\sum_{k=1}^nk!=(n+1)!$
\wrong $1^2+2^2+\cdots+n^2=(n+1)(n+2)(n+3)$
