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S9 = 9/2 (18+8b)
225 = 81+36b
36b = 225-81
b = 4
Un = a + (n-1)b
U7 = a + 6*4
38 = a + 24
a = 38 - 24
a = 14 <- suku pertama
U1 = a+0b = 14
U2 = 14+4 = 18
U9 = 14+9*4 = 50