Решите системы уравнений :
1)[tex]\left \{ {{7x-y^2=-18 } \atop {-5x+y^2=20}} ight.[/tex]
2)[tex]\left \{ {{x-y=5} \atop {x^2+2xy-y^2=-7}} ight.[/tex]
3)[tex]\left \{ {{\frac{4}{x-y}+\frac{12}{x+y}=3 } \atop {\frac{8}{x-y} -\frac{18}{x+y}=-1 }} ight.[/tex]