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-16 = -4r^2
4 = r^2
r = 2
Sn = {a(r^n - 1)}/ r-1
S4= {-4(2^4-1)}/ 2-1
S4 = {-4(31)}
S4 = -124
U1 = a = -4
U3 = -16
DIT : S4 = ?
JAWAB :
U3 = ar²
-16 = -4r²
-16/-4 = r²
4 = r²
√4 = r
r = 2
S4 = a(rn - 1)
r-1
= -4 ( 2^{4} -1 )
2-1
= -4(16-1)
= -4(15)
= -60