Posted by **Robo123** on Saturday, December 11, 2010 at 10:24am.

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

- Math - Geometric Series -
**Reiny**, Saturday, December 11, 2010 at 10:47am
r = -64/128 = -1/2

a = 128

sum(8) = 128( 1 - (1/2)^8 / (1+1/2)

= 128(255/256)(2/3) = 85

- Math - Geometric Series -
**drwls**, Saturday, December 11, 2010 at 10:57am
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

- Math - Geometric Series -
**sylow theorems**, Sunday, December 12, 2010 at 10:08pm
128-64+32-16+8-4+2-1=64+16+4+1=80+5=85(!)

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