21. Let $S_{n}$ be the sum of the first $n$ terms of an arithmetic sequence. Assume that $S_{3}=9, S_{20}$ $>0$, and $S_{21}<0$. Then the range of the common difference $d$ is $\qquad$ , the maximum term of the sequence $S_{1}, S_{2}, S_{3}, \cdots$ is $\qquad$

Note: term - one item, arithmetic sequence - arithmetic sequence, common difference - common difference, maximum term - maximum (value) term
$\equiv 21 \cdot\left(-\frac{6}{17},-\frac{1}{3}\right), S_{10}$.
The problem states: Let $S_{n}$ represent the sum of the first $n$ terms of an arithmetic sequence, $S_{3}=9, S_{20}>0, S_{21}0$,
$S_{21}=21 a_{1}+210 d0, a_{11}<0$.
Therefore, starting from the 11th term, it is less than 0. Hence, $S_{10}$ is the maximum.