Ta có :
\(\frac{2011}{2012+2013}< \frac{2011}{2012}\)
\(\frac{2012}{2012+2013}< \frac{2012}{2013}\)
\(\Rightarrow\frac{2011}{2012+2013}+\frac{2012}{2012+2013}< \frac{2011}{2012}+\frac{2012}{2013}\)
\(\Rightarrow\frac{2011+2012}{2012+2013}< B\)
\(\Rightarrow A< B\)
Vậy \(A< B\)