1/2 + 1/4 + 1/8 + 1/16 +1/32 + 1/64 + 1/128
=1-1/2+1/2-1/4+1/4-1/8+...+1/64+1/128
=1-1/128
=127/128
\(\frac{1}{2}+\frac{1}{4}+\frac{1}{8}+\frac{1}{16}+\frac{1}{32}+\frac{1}{64}+\frac{1}{128}\)
\(=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{4}+\frac{1}{4}-\frac{1}{8}+...+\frac{1}{64}-\frac{1}{128}\)
\(=1-\frac{1}{128}\)
\(=\frac{127}{128}\)
\(\frac{1}{2}+\frac{1}{4}+\frac{1}{8}+\frac{1}{16}+\frac{1}{32}+\frac{1}{64}+\frac{1}{128}\)
= \(1-\frac{1}{2}+\frac{1}{2}-\frac{1}{4}+\frac{1}{4}-\frac{1}{8}+\frac{1}{8}-...........+\frac{1}{64}-\frac{1}{64}+\frac{1}{128}\)
= \(1-\frac{1}{128}\)
= \(\frac{127}{128}\)
S=1/2+1/4+....+1/128
=>2S=1+1/2+1/4+...+1/64
=>2S-S=(1+1/2+1/4/...1/64)-(1/2+1/4+...+1/128)
=>S=1-1/128
=>S=127/128
tính nhanh p/s 1+ 5/4 + 5/8 + 5/16 + 5/32 + 5/64
