[填空题]
The 15th term of an arithmetverk2: uxs,ehl.g u55ic sequence is 21 and the comm w :dncnz8g.if5/xkpt 6)eb3 dq- id9con difference is −4.
Find the first term of the sequence.
$u_1$= .
Find the 29th term of the sequence.
$u_29$=- .
Find the sum of the first 40 terms of the sequence.
$S_{40}$=- .
