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rumusnya Sn = n/2 (a+Un)
S11= 11/2 (a+U11) , inget rumus Un = a+(u-1)b
110 = 11/2 (30+(30+(11-1)b))
110 = 11/2 (30+30+10b)
110 = 11.30/2 + 11.30/2 + 11.10b/2
110 = 165 + 165 + 55b
110 = 330 + 55b
110-330 = 55b
-220/55 = b
-4 = b , jadi beda atau b = (-4)
Sn=n/2(2a+(n-1)b)
110=11/2(2a+10b)
10=a+5b
10=30+5b
b=-4