Respuesta :

sqdancefan
The formula for the sum of a geometric sequence with first term "a", common ratio "r" and number of terms "n" is
  S = a·(r^n -1)/(r -1)
You have a=2, r=-2, n=7, so the sum is
  S = 2·((-2)^7 -1)/(-2 -1)
  = 2(-128-1)/(-3)
  = 2(43) = 86
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