[tex]\displaystyle 1)\left \{ {{2x-y=5} \atop {2x+y=7}} \right. \\\\\left \{ {{2x-y=5} \atop {2x=7-y}} \right. \\\\7-y-y=5\\y=1\\2x=7-1\\x=3\\(x,y)=(3,1)\\\\2)\left \{ {{4x-y=5} \atop {2x+y=1}} \right.\\ \\(4x-y)+(2x+y)=5+1\\6x=6\\x=1\\4*1-y=5\\y=-1\\(x,y)=(1,-1)\\\\3)\left \{ {{3x-4y=18} \atop {5x+2y=4}} \right. \\\\(3x-4y)+2(5x+2y)=18+2*4\\13x=26\\x=2\\3*2-4y=18\\y=-3\\(x,y)=(2,-3)[/tex]