u3=20 u8= 40 u30=....

u3 = a + 2b -> 20 = a + 2b ...(1)
u8 = a + 7b -> 40 = a + 7b.....(2)

eliminasi persamaan (1) dan (2)
20 = a + 2b
40 = a + 7b
_________-
-20 = -5b
b = 4

substitusi b ke persamaan (1)
20 = a + 2b
20 = a + 2(4)
a = 12

u30 = a + 29b
u30 = 12 + 29(4) = 128

jadi, u30 = 128

u8 = 40 = a + 7b
u3 = 20 = a + 2b

Kurangkan sehingga menjadi:

20 = 5b
b = 4

Substitusi:
20 = a + 2b
20 = a + 2(4)
20 = a + 8
a = 20 - 8
a = 12

Penyelesaian:
u30 = a + 29b
u30 = 12 + 29(4)
u30 = 12 + 116
u30 = 128