\[\boxed{\text{683}\text{\ (683)}\text{.}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[\textbf{а)}\ a_{20} = 1,7,\ \ a_{37} = 0,\ \ d = ?\]
\[a_{37} = a_{20} + 16d\]
\[0 = 1,7 + 17d\]
\[17d = - 1,7\]
\[d = - 0,1.\]
\[\textbf{б)}\ a_{10} = 270,\ \ d = - 3,\ \ a_{100} = ?\]
\[a_{100} = a_{10} + 90d = 270 - 3 \cdot 90 = 0.\]