2/3 + 1/3 + 0 ... sampai suku ke-18

2/3 + 1/3 + 0 + ....
a=2/3
b=1/3-2/3=-1/3
S18=18/2(2a + (n-1)b)
=9(2(2/3) + (18-1)(-1/3))
=9(4/3-17/3)
=9(-13/3)
=3.(-13)=-39

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