[填空题]
A battalion is arranged, per row, according to an arc8wsqo) t/q,d 0xhiv9 f3 4u uw,z)ysaithmet+gp/ 9xtt, i wzpmta21ic sequence. There are2p1pit ,/m zxatt9+ wg 24 troops 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.
