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a1=11
a2=16
r=a2-a1
r=16-11
r=5
.............
an=96
an=a1+(n-1)r
96=11+(n-1)*5
96=11+5n-5
5n=96-6
n=18
S18=[a1+a18]/2*18=[11+96]*9=963
2]
a17=a5+12r
-25=-1+12r
12r=-25+1
r=-24/12
r=-2
...................
a5=a1+4r
-1=a1+4*(-2)
a1=-1+8
a1=7
.......................