so sanh A va B A=(1/2 ² -1).(1/3 ² -1).(1/4 ²-1).......(1/2014 ²-1) B= -1/2

$\begin{array}{l} A = \left( {\frac{1}{{{2^2}}} - 1} \right)\left( {\frac{1}{{{3^2}}} - 1} \right)\left( {\frac{1}{{{4^2}}} - 1} \right)......\left( {\frac{1}{{{{2014}^2}}} - 1} \right)\\ Trong\,\,\,bieu\,\,thuc\,\,A\,\,co\,\,tat\,ca:\,\,\,\frac{{2014 - 2}}{2} + 1 = 1007\,\,phan\,\,tu.\\ \Rightarrow A\,\,co\,\,gia\,\,tri\,\,am\,\,hay\,\,A < 0.\\ \Rightarrow A = \left( {\frac{1}{2} - 1} \right)\left( {\frac{1}{2} + 1} \right)\left( {\frac{1}{3} - 1} \right)\left( {\frac{1}{3} + 1} \right)\left( {\frac{1}{4} - 1} \right)\left( {\frac{1}{4} + 1} \right).......\left( {\frac{1}{{2014}} - 1} \right)\left( {\frac{1}{{2014}} + 1} \right)\\ = - \left[ {\left( {1 - \frac{1}{2}} \right)\left( {1 - \frac{1}{3}} \right)\left( {1 - \frac{1}{4}} \right)......\left( {1 - \frac{1}{{2014}}} \right)} \right]\left[ {\left( {1 + \frac{1}{2}} \right)\left( {1 + \frac{1}{3}} \right)\left( {1 + \frac{1}{4}} \right)......\left( {1 + \frac{1}{{2014}}} \right)} \right]\\ = - \left( {\frac{1}{2}.\frac{2}{3}.\frac{3}{4}.....\frac{{2013}}{{2014}}} \right).\left( {\frac{3}{2}.\frac{4}{3}.\frac{5}{4}.....\frac{{2015}}{{2014}}} \right)\\ = - \frac{1}{{2014}}.\frac{{2015}}{2} = - \frac{1}{2}.\frac{{2015}}{{2014}} < - \frac{1}{2}.\\ \Rightarrow A < B. \end{array}$