Ta có :
\(S=\frac{1}{2}+\frac{1}{2^2}+\frac{1}{2^3}+...+\frac{1}{2^{2013}}\)
\(2S=1+\frac{1}{2}+\frac{1}{2^2}+...+\frac{1}{2^{2012}}\)
\(2S-S=\left(1+\frac{1}{2}+\frac{1}{2^2}+...+\frac{1}{2^{2012}}\right)-\left(\frac{1}{2}+\frac{1}{2^2}+\frac{1}{2^3}+...+\frac{1}{2^{2013}}\right)\)
\(S=1-\frac{1}{2^{2013}}\)
\(S=\frac{2^{2013}-1}{2^{2013}}\)
Vì \(\frac{2^{2013}-1}{2^{2013}}< 1\) ( tử bé hơn mẫu nên bé hơn 1 ) nên \(S< 1\)
Vậy \(S< 1\)