[填空题]
The 15th term of an r)z t1,ah uk:p6iue *ncjj 5de lb:e/a,.e 6k6t g5czoarithmetic sequence is 21 and 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.
$S_{40}$=- .
