Math  Geometric Series
posted by Robo123 on .
Find the sum of the geometric series 128  64 + 32  . . . to 8 terms.
a) 85
b) 225
c) 86
d) 85/2
e) not possible

r = 64/128 = 1/2
a = 128
sum(8) = 128( 1  (1/2)^8 / (1+1/2)
= 128(255/256)(2/3) = 85 
With only 8 terms, it's easier for me to do the addition than remember the formula.
Adding in pairs, successive pairs are lower by a factor of 4.
64 + 16 + 4 + 1 = 85 
12864+3216+84+21=64+16+4+1=80+5=85(!)