\(A=\frac{1}{2}+\frac{2}{2^2}+\frac{3}{2^3}+...+\frac{2014}{2^{2014}}\)
\(\Rightarrow2A=1+\frac{2}{2}+\frac{3}{2^2}+...+\frac{2014}{2^{2013}}\)
\(\Rightarrow2A-A=\left(1+\frac{2}{2}+\frac{3}{2^2}+...+\frac{2014}{2^{2013}}\right)-\left(\frac{1}{2}+\frac{2}{2^2}+...+\frac{2014}{2^{2014}}\right)\)
\(\Rightarrow A=1+\frac{1}{2}+\frac{1}{2^2}+...+\frac{1}{2^{2013}}-\frac{2014}{2^{2014}}\)
Đặt \(B=1+\frac{1}{2}+\frac{1}{2^2}+...+\frac{1}{2^{2013}}\)
\(\Rightarrow2B=2+1+...+\frac{1}{2^{2012}}\)
\(\Rightarrow2B-B=\left(2+1+...+\frac{1}{2^{2012}}\right)-\left(1+\frac{1}{2}+...+\frac{1}{2^{2013}}\right)\)
\(\Rightarrow B=2-\frac{1}{2^{2013}}< 2\)
\(\Rightarrow B< 2\)
\(\Rightarrow A< 2-\frac{2014}{2^{2014}}< 2\)
\(\Rightarrow A< 2\left(đpcm\right)\)
