tolong kerjakan no 6

Sn = 2n²-n
n/2 . [2a+(n-1)b] = 2n²-n
an+n(n-1)b/2 = 2n²-n
an+(bn²-bn)/2 = 2n²-n
2an+bn²-bn = 4n²-2n
<> b=4
<> 2a-b= -2 => 2a-4= -2 => 2a=2 => a=1
Un=a+(n-1)b
U20=1+(20-1)4=77 (B)