a9=5 d=2
a8=5-2=3
a9=a1+8*d
5=a1+ 8*2
a1=5-16
a1=-11
S= (a1+a8)/2 *8
S=(-11+3)/2 *8=-32
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a9=5 d=2
a8=5-2=3
a9=a1+8*d
5=a1+ 8*2
a1=5-16
a1=-11
S= (a1+a8)/2 *8
S=(-11+3)/2 *8=-32