10: For $n\in\{0,1,2,\ldots\}$ define $\tau(n)=n(n+1)/2$.
14: For each $n\geq0$ one has $\sum_{j=1}^n j=\tau(n)$.
18: adding $n+1$ gives $(n+1)(n+2)/2=\tau(n+1)$.
