a)[tex]+\left \{ {{x+y=2} \atop {-x+y=2}} \right. \\------\\2y=4 /:2\\y=2\\x+y=2\\x+2=2\\x=2-2\\x=0\\\left \{ {{x=0} \atop {y=2}} \right.[/tex]
b)[tex]\left \{ {{-x+y=-1/*2} \atop {-2x+3y=1}} \right. \\-\left \{ {{-2x+2y=-2} \atop {-2x+3y=1}} \right. \\--------\\-y=-3/:(-1)\\y=3\\-x+y=-1\\-x+3=-1\\-x=-1-3\\-x=-4/:(-1)\\x=4\\\left \{ {{x=4} \atop {y=3}} \right.[/tex]
c)[tex]-\left \{ {{3x+y=5} \atop {\frac{1}{2}x+y=0 }} \right. \\------\\2\frac{1}{2} x=5\\\frac{5}{2}x=5/:\frac{5}{2}\\x =5*\frac{2}{5} \\x = 2\\3x+y=5\\3*2+y=5\\6+y=5\\y=5-6\\y=-1\\\left \{ {{x=2} \atop {y=-1}} \right.[/tex]
