[tex]a)\\\left \{ {{5x+2y=7} \atop {3x-y=13 /*2}} \right. \\+\left \{ {{5x+2y=7} \atop {6x-2y=26}} \right. \\5x+6x+2y-2y=7+26\\11x=33 /:11\\x=3\\5*3+2y=7\\15+2y=7 /-15\\2y=-8 /:2\\y=-4[/tex]
[tex]b) \\\left \{ {{4x+3y=9} \atop {-x+5y=2 /*4}} \right. \\+\left \{ {{4x+3y=9} \atop {-4x+20y=8}} \right. \\4x-4x+3y+20y=9+8\\23y=17 /:23\\y=\frac{17}{23}\\-x+5*\frac{17}{23}=2\\-x+\frac{85}{23}=2 /-\frac{85}{23}\\-x=\frac{46}{23}-\frac{85}{23}\\-x=-\frac{39}{23}\\x=\frac{39}{23}[/tex]
[tex]\left \{ {{x+7y=12 /*(-3)} \atop {3x-5y=10}} \right. \\+\left \{ {{-3x-21y=-36} \atop {3x-5y=10}} \right. \\-3x+3x-21y-5y=-36+10\\-26y=-26 /:(-26)\\y=1\\x+7*1=12\\x+7=12 /-7\\x=5[/tex]
