是两元一次方程组{6(x-y)-7(x+y)=212(x-y)-5(x+y)=-1

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是两元一次方程组{6(x-y)-7(x+y)=212(x-y)-5(x+y)=-1

是两元一次方程组{6(x-y)-7(x+y)=212(x-y)-5(x+y)=-1
是两元一次方程组
{6(x-y)-7(x+y)=21
2(x-y)-5(x+y)=-1

是两元一次方程组{6(x-y)-7(x+y)=212(x-y)-5(x+y)=-1
2(x-y)-5(x+y)=-1 乘3 得 6(x-y)-15(x+y)=-3
6(x-y)-7(x+y)=21 减 6(x-y)-15(x+y)=-3 得 8(x+y)=24 ,(x+y)=3
把(x+y)=3 代入 6(x-y)-7(x+y)=21 得 (x-y)=7
然后(x+y)=3 加上(x-y)=7 得x=5 ,y=-2