SI. 4 The sequence $\left\{a_{n}\right\}$ is defined as $a_{1}=R, a_{n+1}=a_{n}+2 n(n \geq 1)$. If $a_{100}=S$, find the value of $S$.
$\begin{array}{l}a_{1}=2, a_{2}=2+2, a_{3}=2+2+4, \ldots \\ \begin{aligned} a_{100} & =2+2+4+\ldots+198 \\ & =2+\frac{1}{2}(2+198) \cdot 99=9902=S\end{aligned}\end{array}$