{4(x+2)=1-5y,3(y+2)=3-2x {2/y+3/x=13,3/y-4/x=3 {x+y=420,30%x
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{4(x+2)=1-5y,3(y+2)=3-2x {2/y+3/x=13,3/y-4/x=3 {x+y=420,30%x+40%y=160*80% 用加减消元法解方程组
{4(x+2)=1-5y,3(y+2)=3-2x
4x+8=1-5y
4x+5y=-7 (1)
3(y+2)=3-2x
3y+6=3-2x
2x+3y=-3 (2)
(2)*2-(1)
4x+6y-4x-5y=-6+7
y=1
x=-3
{2/y+3/x=13 (1)
3/y-4/x=3 (2)
(1)*4+(2)*3
8/y+12/x+9/y-12/x=13*4+3*3
17/y=61
y=17/61
x=17/33
{x+y=420 (1)
30%x+40%y=160*80%
3x+4y=1280 (2)
(1)*3-(2)
3x+3y-3x-4y=420*3-1280
y=20
x=400
4x+8=1-5y
4x+5y=-7 (1)
3(y+2)=3-2x
3y+6=3-2x
2x+3y=-3 (2)
(2)*2-(1)
4x+6y-4x-5y=-6+7
y=1
x=-3
{2/y+3/x=13 (1)
3/y-4/x=3 (2)
(1)*4+(2)*3
8/y+12/x+9/y-12/x=13*4+3*3
17/y=61
y=17/61
x=17/33
{x+y=420 (1)
30%x+40%y=160*80%
3x+4y=1280 (2)
(1)*3-(2)
3x+3y-3x-4y=420*3-1280
y=20
x=400
{3(x+y)-4(x-y)=4 {x+y/2 + x-y/6=1
3(x+y)-4(x-y)=11,2(x-y)+5(x+y)=27
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x+y/2+x-y/3=6,4(x+y)-3(x-y)=-20
{(x+y)/2+(x-y)/3=6 4(x+y)-3(x-y)=-20
4(x+y)-3(x-y)=-20,2/x+y+3/x-y=6
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