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a6=a1+5r
a11=a1+10r
a11-a6=10r-5r
5r=50-5,5
r=44,5/5
r=8,9
a1=a6-5r
a1=5,5-44,5
a1=-39
a20=-39+19*8,9 =-39+169,1
a20=130,1
Sn=(a1+an)*n/2
S20=(-39+130,1)*20/2
S20=91,1*10
S20=911