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Verified answer
Sn = n/2 { 2a + (n-1)b }S11 = 11/2 { 2.30 + (11 -1).b
110 = 11/2 { 60 + 10b)
220 = 11 ( 60 + 10b)
20 = 60 + 10b
10b = -40
b = -4
Jadi beda b = -4