7. The numbers $3,6,10,15, \cdots$ are called triangular numbers because these numbers of points can be arranged in a regular triangle, as shown in Figure 2. Then the 19th triangular number is $\qquad$
$=, 7.210$.
It is easy to归纳 summarize that $a_{n}=\frac{(n+1)(n+2)}{2}\left(n \in \mathbf{Z}_{+}\right)$. Therefore, $a_{19}=\frac{20 \times 21}{2}=210$.

Note: The word "归纳" in the original text is a Chinese term that means "summarize" or "induction." I have replaced it with "summarize" to maintain the flow in English. If you prefer to keep the original term, please let me know.