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6r=8-(-4)=8+4=12
r=2
Sn=1/2*(2a1+r(n-1))*n
S20=1/2*(2*(-4)+2*19)*20=20*(-4+19)=20*15=300
a₇=8
a₇=a₁+6r
8=-4+6r
12=6r
r=2