[填空题]
The 15th term of an arithmetic sequence i,.8qd 6 ,6x:hwkxodrm4.iizqgb g ;p9o t3yhls 21 and the4dv hsv0: hv *bnc3:oi 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.
$S_{40}$=- .
