Đặt A
A= 1/2 + 1/4 + 1/8 + 1/16 + 1/32 + 1/64 + 1/128 + 1/256
2A= 2(1/2 + 1/4 + 1/8 + 1/16 + 1/32 + 1/64 + 1/128 + 1/256)
= 1+1/2 + 1/4 + 1/8 + 1/16 + 1/32 + 1/64 + 1/128
=>A = 2A-A =1+1/2 + 1/4 + 1/8 + 1/16 + 1/32 + 1/64 + 1/128 -1/2 - 1/4 - 1/8 - 1/16 - 1/32 - 1/64 - 1/128 - 1/256
=1-1/256
=255/256
Chúc học tốt 
Đặt A = 1/2+1/4+1/8+1/18+1/32+1/64+1/128+1/256
=> 2A = 1+1/2+1/4+1/8+1/18+1/32+1/64+1/128
=>2A - A = 1 - 1/256
=> A = 255/256
1/2+1/4+1/8+1/18+1/32+1/64+1/128+1/256 \(=\frac{2279}{2304}\)
nha bạn 
^_^
\(\frac{1}{2}+\frac{1}{4}+\frac{1}{8}+\frac{1}{32}+\frac{1}{64}+\frac{1}{128}+\frac{1}{256}\)
=>\(2A=1+\frac{1}{2}+\frac{1}{4}+\frac{1}{8}+\frac{1}{18}+\frac{1}{32}+\frac{1}{64}+\frac{1}{128}+\frac{1}{256}\)
\(2A-A=1+\frac{1}{2}+\frac{1}{4}+\frac{1}{8}+\frac{1}{18}+\frac{1}{32}+\frac{1}{64}+\frac{1}{128}\)-\(\frac{1}{2}-\frac{1}{4}-\frac{1}{8}-\frac{1}{18}-\frac{1}{32}-\frac{1}{64}-\frac{1}{128}-\frac{1}{256}\)
=>\(A=1-\frac{1}{256}=\frac{255}{256}\)
tính nhanh p/s 1+ 5/4 + 5/8 + 5/16 + 5/32 + 5/64
