$\frac{1}{4}$+ $\frac{1}{8}$+ $\frac{1}{16}$+ $\frac{1}{32}$+ $\frac{1}{64}$+ $\frac{1}{128}$
Áp dụng công thức $\frac{1}{k}$ =$\frac{1}{k:2}$- $\frac{1}{k}$
⇒ $\frac{1}{4}$ +$\frac{1}{8}$+ $\frac{1}{16}$ +$\frac{1}{32}$ +$\frac{1}{64}$+ $\frac{1}{128}$
=$\frac{1}{2}$- $\frac{1}{4}$+ $\frac{1}{4}$- $\frac{1}{8}$ +$\frac{1}{8}$ -$\frac{1}{16}$ +$\frac{1}{16}$- $\frac{1}{32}$+ $\frac{1}{32}$ -$\frac{1}{64}$+ $\frac{1}{64}$- $\frac{1}{128}$
=$\frac{1}{2}$+0+0+...+0- $\frac{1}{128}$
= $\frac{1}{2}$ -$\frac{1}{128}$
=$\frac{63}{128}$