[填空题]
The 15th term of an arithmetu4ejyr::(n;i q /lll qic sequence is 21 andcbd 4bir efm9 u *usi86:rl+yzgw-p3 ) the common 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.
$S_{40}$=- .
