Posted by Robo123 on Saturday, December 11, 2010 at 10:24am.
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
128-64+32-16+8-4+2-1=64+16+4+1=80+5=85(!)
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