Three, in an arithmetic sequence, if the 11th term is 20, find the sum of the first 21 terms.
Three, Solution From $a_{1}+10 \mathrm{~d}=20$, we have
$$
S_{21}=21 a_{1}+\frac{21 \cdot 20}{2} d=21\left(a_{1}+10 d\right)=420 .
$$