{6(x-y)-7(x+y)=21 ① {2(x-y)-5(x+y)=-1 ②这是方程组

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{6(x-y)-7(x+y)=21①{2(x-y)-5(x+y)=-1②这是方程组{6(x-y)-7(x+y)=21①{2(x-y)-5(x+y)=-1②这是方程组{6(x-y)-7(x+y)=21①

{6(x-y)-7(x+y)=21 ① {2(x-y)-5(x+y)=-1 ②这是方程组
{6(x-y)-7(x+y)=21 ① {2(x-y)-5(x+y)=-1 ②
这是方程组

{6(x-y)-7(x+y)=21 ① {2(x-y)-5(x+y)=-1 ②这是方程组
{6(x-y)-7(x+y)=21.1
{2(x-y)-5(x+y)=-1.2
1式-2式*3得
-7(x+y)-[-3*5(x+y)]=21-(-1*3)
-7(x+y)+15(x+y)=24
8(x+y)=24
x+y=3.3
将3式代入1式得
6(x-y)-7*3=21
6(x-y)-21=21
6(x-y)=42
x-y=7.4
3式+4式得
2x=10
x=5
3式-4式得
2y=-4
y=-2
所以x=5,y=-2

把(x-y),(x+y)看作整体
计算
x-y=7
x+y=3
再计算
x=5
y=-2

(1)-(2)*3得 8(x+y)=24, x+y=3 (3)
(3)代入(1)化简得 x-y=7 (4)
(3)+(4) 得 2x=10,x=5
(3)-(4) 得2y=-4, y=-2

先化简第一个,6X-6Y-7X-7Y=21
-X-13Y=21
X+13Y=-21.
X=-21-13Y
化简第二个2X-2Y-5X-5Y=-1
-3X-7Y=-1
3X+7Y=1
将一式化简结果带入二式化简结果中
3*(-21-13Y)+7Y=1
化简-63-39Y+7Y=1
解Y=-2.将Y值带入一试化简结果中。X=-21-13*(-2)=5

令x-y=m,x+y=n
则解方程组6m-7n=21①,2m-5n=-1②得
m=7,n=3
再解方程组x-y=7③,x+y=3④得
x=5,y=-2.

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