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beda = 3
Un(terakhir) = 108
Un = a +(n-1)b
Un = 21 + (n-1)3
Un = 21 + 3n - 3
Un = 3n + 18
108 = 3n + 18
3n = 108-18
3n = 90
n = 90/3
n = 30
Sn = n/2 (a+Un)
S30 = 30/2 (21 + 108)
S30 = 15 (129)
S30 = 1935