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a + 18b = 74
------------------- -
7b = 28
b = 4
a = 2
U30 = 2 + 29 x 4 = 2 + 116 = 118
S30 = 30/2 x (2 + 118)
S30 = 15 x 120 = 1800
Bonus rumus:
Un = a + (n - 1) x b
Sn = n/2 x (a + Un)