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U3 = a + (3-1) b
U8 = a + (8-1) b
____________
4 = a + 2b
-11= a + 7b
_________-
15 = -5b
b = -3
4 = a + 2b
4 = a + 2 (-3)
4 = a- 6
a = 10
Jadi, U20 = a + (n-1) b
= 10 + 19 (-3)
= 10 - 57 = -47