-2 + 4 + -8 + ... Tentukan U8 dan S12

-2 + 4 + -8 + ...
a=-2
r= 4/-2= -2

U8? S12?

Un = ar^[n-1]
U8 = -2(-2)^[8-1]
      = 4^[7]
      = 16.384

Sn = a (1 - r^[n]) / 1-r
S12= -2 (1- (-2)^[12]) / 1-(-2)
       = -2 ( 1 - 4096) / 3
       = -2 (-4095) / 3
       = 8190 / 3
       = 2730