Ta có:
\(\frac{1}{2}+\frac{1}{4}+\frac{1}{8}+...+\frac{1}{16}+\frac{1}{28}\)
\(=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{4}+...+\frac{1}{8}-\frac{1}{16}+\frac{1}{28}\)
\(=1-\frac{1}{16}+\frac{1}{28}\)
\(=\frac{15}{16}-\frac{1}{28}\)
\(=\frac{105}{112}-\frac{4}{112}\)
\(=\frac{101}{112}\)
2A=1+1/2+1/4+...+1/8+1/16
2A-A=1-1/28
=>A=27/28