Explanation:
this sequence has 8 terms and the common ratio or multiplication factor is -1/2.
we could add these 8 numbers manually or use the formula :
Sn = a1(1 - r^n)/(1 - n)
when r < 1 (like in our case -1/2).
S8 = 128(1 - (-1/2)⁸)/(1 - -1/2) =
= 128(1 - 1/256)/(3/2) = (128×255/256)/(3/2) =
= (255/2)/(3/2) = 255/2 × 2/3 = 255/3 = 85