a1=-28
an=a1+(n-1)d
n=9
Sn=0
Sn=((a1+an)/2)n
0=((an-28)/2)*9
an=28
28=-28+(9-1)d
8d=56
d=7
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a1=-28
an=a1+(n-1)d
n=9
Sn=0
Sn=((a1+an)/2)n
0=((an-28)/2)*9
an=28
28=-28+(9-1)d
8d=56
d=7