Jawabanmu

2014-03-05T20:21:06+07:00
U2= ar = -3 -----> a= -3/r
U5= ar^4 = -81

maka :
-3/r x r^4 = -81
r^3 = 27
r = 3

a = -3/r
a= -3/3 = -1

S8 = a (r^n-1)/r-1 
     = -1 (3^8 -1) / 2
     = -1 (6561-1)/2
     = -3280
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