Решите системы ура-ний: 1) [tex] \left \{ {{x+2y=3} \atop { x^{2} -2xy+4 y^{2} =21}} \right. [/tex]
2) [tex] \left \{ {{ \frac{9}{x+y}+ \frac{2}{x-y} =3} \atop { \frac{18}{x+y}- \frac{5}{x-y} =-3}} \right. [/tex]
3) [tex] \left \{ {{3x-2y=6} \atop { x^{2} -4y=4}} \right. [/tex]