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Verified answer
Materi : Barisan dan DeretKelas : IX SMP
U3 = U1 + 2b = 9
U8 = U1 + 7b = 4
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⇒⇒⇒⇒⇒ -5b = 5
⇒⇒⇒⇒⇒ b = -1
U3 = U1 + 2b
9 = U1 + 2(-1)
9 = U1 - 2
U1 = 9 + 2
U1 = 11
U20 = U1 + 19b
U20 = 11 + 19(-1)
U20 = 11 - 19
U20 = -8
a + 2b = 9 . . . (i)
U8 = 4
a + 7b = 4 . . . (ii)
(i) & (ii)
a + 2b = 9
a + 7b = 4
-------------- -
-5b = 5
b = -1
a + 2b = 9
a + 2(-1) = 9
a - 2 = 9
a = 11
U20 = a + 19b = 11 + 19(-1) = 11 - 19 = -8