Bài giải

A =$\frac{1}{2}+\frac{1}{4}+\frac{1}{8}+\frac{1}{16}+\frac{1}{32}+\frac{1}{64}+\frac{1}{128}$

A = $\frac{1\times 64}{2\times 64}$ + $\frac{1\times 32}{4\times 32}$ + $\frac{1\times 16}{8\times 16}$ + $\frac{1\times 8}{16\times 8}$ + $\frac{1\times 4}{32\times 4}$ + $\frac{1\times 2}{64\times 2}$ + $\frac{1}{128}$

A = $\frac{64}{128}$ + $\frac{32}{128}$ + $\frac{16}{128}$ + $\frac{8}{128}$ + $\frac{4}{128}$ + $\frac{2}{128}$ + $\frac{1}{128}$

A = $\frac{127}{128}$