giải hệ$1)\begin{cases}\frac{x+\sqrt{x^{2}-y^{2}}}{x-\sqrt{x^{2}-y^{2}}}=\frac{9x}{5} \\ \frac{x}{y}=\frac{5+3x}{30-6y} \end{cases}$$2)\begin{cases}\sqrt{2y}\left ( 3-\frac{5}{y+42x} \right )=4 \\ \sqrt{x}\left ( 3-\frac{5}{x+42y} \right )=2 \end{cases}$$3) \begin{cases}4xy+4(x^{2}+y^{2})+\frac{3}{x^{2}+y^{2}}=7 \\ 2x+\frac{1}{x+y}=3 \end{cases}$