[填空题]
A battalion is arran0 jvlx1on -(feged, per row,;up 372romi ti according to an arithmetic sequence. There are 24 troops72i ;irou ptm3 in the third row and 42 troops in the sixth row.
1.Find the first term and the common difference of this arithmetic sequence.
$u_1$= .
There are 15 rows in the battalion.
2.Find the total number of troops in the battalion.
$S_{15}$= troops.
