$\displaystyle \begin{array}{{>{\displaystyle}l}} A=\frac{2x+5y}{2x-5y}\\ \Rightarrow A^{2} =\frac{4x^{2} +25y^{2} +20xy}{4x^{2} +25y^{2} -20xy} =\frac{30xy+20xy}{30xy-20xy} =5\\ Do\ 2x< 5y< 0\Rightarrow ( 2x+5y) < 0\ và\ ( 2x-5y) < 0\\ \Rightarrow A >0\\ \Rightarrow A=\sqrt{5} \end{array}$