[填空题]
The 15th term of an y0.gdy,1 w*m7e )r.ma8r ird gavjir2a6c8f(ivchniy o00.d e rithmetic 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.
$S_{40}$=- .
