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Сначала из 1 уравнения вычтем 2-ое. А затем сложим два уравнения.
![left { {{y^2+xy=4} atop {x^2+xy=-3}} right. ; left { {{y^2-x^2=7} atop {x^2+2xy+y^2=1}} right. ; left { {{(y-x)(y+x)=7} atop {(x+y)^2=1}} right. ; left { {{(y-x)cdot 1=7; ili; (x-y)cdot (-1)=7} atop {x+y=1; ili; x+y=-1}} right. \\a); left { {{y-x=7} atop {x+y=1}} right. ; left { {{2y=8} atop {2x=-6}} right. ; left { {{y=4} atop {x=-3}} right. \\b); left { {{-(x-y)=7} atop {x+y=-1}} right. ; left { {{y-x=7} atop {x+y=-1}} right. left { {{y^2+xy=4} atop {x^2+xy=-3}} right. ; left { {{y^2-x^2=7} atop {x^2+2xy+y^2=1}} right. ; left { {{(y-x)(y+x)=7} atop {(x+y)^2=1}} right. ; left { {{(y-x)cdot 1=7; ili; (x-y)cdot (-1)=7} atop {x+y=1; ili; x+y=-1}} right. \\a); left { {{y-x=7} atop {x+y=1}} right. ; left { {{2y=8} atop {2x=-6}} right. ; left { {{y=4} atop {x=-3}} right. \\b); left { {{-(x-y)=7} atop {x+y=-1}} right. ; left { {{y-x=7} atop {x+y=-1}} right.](https://tex.z-dn.net/?f=+left+%7B+%7B%7By%5E2%2Bxy%3D4%7D+atop+%7Bx%5E2%2Bxy%3D-3%7D%7D+right.+%3B++left+%7B+%7B%7By%5E2-x%5E2%3D7%7D+atop+%7Bx%5E2%2B2xy%2By%5E2%3D1%7D%7D+right.+%3B++left+%7B+%7B%7B%28y-x%29%28y%2Bx%29%3D7%7D+atop+%7B%28x%2By%29%5E2%3D1%7D%7D+right.+%3B++left+%7B+%7B%7B%28y-x%29cdot+1%3D7%3B+ili%3B+%28x-y%29cdot+%28-1%29%3D7%7D+atop+%7Bx%2By%3D1%3B+ili%3B+x%2By%3D-1%7D%7D+right.+%5C%5Ca%29%3B++left+%7B+%7B%7By-x%3D7%7D+atop+%7Bx%2By%3D1%7D%7D+right.+%3B++left+%7B+%7B%7B2y%3D8%7D+atop+%7B2x%3D-6%7D%7D+right.+%3B++left+%7B+%7B%7By%3D4%7D+atop+%7Bx%3D-3%7D%7D+right.+%5C%5Cb%29%3B++left+%7B+%7B%7B-%28x-y%29%3D7%7D+atop+%7Bx%2By%3D-1%7D%7D+right.+%3B++left+%7B+%7B%7By-x%3D7%7D+atop+%7Bx%2By%3D-1%7D%7D+right.+)
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