{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

{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 已知x-y/x+y=3,求代数式5(x-y)/x+y-x+y/2(x-y) 化简[(3x+4y)^2-(2x+y)(2x-y)+(-x+y)(5x-y)]除以-2y,其中x=-1,y=1 已知x=1/3,y=-1/2,求代数式x-(x+y)+(x+2y)-(x+3y)+(x+4y)-(x+5y)+...-( 先化简再求值(x-y)(x+y)-(x-2y) 的完全平方+x(3x-5y)-(x-y)(x-2y),其中x=1/2 y 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 (x+y)(x+2y)(x+3y)(x+4y)=-40 已知X-Y/X+Y=3,求代数式2(x-y)/X+Y-3X+Y/X+Y 数学题……555{(3x+y)(3x-y)-(x-5y)(5x-y)-(x-2y)²÷(-4x),x=-2.y