\begin{align*} 1 \amp = 1\\ 3 \amp = 1+2\\ 6 \amp = 1 + 2 + 3\\ 10 \amp = 1+2 + 3+ 4\\ \vdots \amp \qquad \vdots\\ T_n \amp = 1 + 2 + 3 + \cdots + n \end{align*}
in-context