$4x^{2}$ +$25y^{2}$ $=30xy$
+)⇒$4x^{2}$ +$25y^{2}$+$20xy$$=30xy+20xy$
⇒$(2x+5y)^{2}$ $=50xy$
+)⇒$4x^{2}$ +$25y^{2}$-$20xy$$=30xy-20xy$
⇒$(2x-5y)^{2}$ $=10xy$
Có $A$=$\frac{2x+5y}{2x-5y}$
⇒$A^{2}$ $=$$\frac{(2x+5y)^{2}}{(2x-5y)^{2}}$ =$\frac{50xy}{10xy}$ =$5$
⇒$A^{2}$=$5$
⇒$A$$=$$±$$√5$