2y-x=5,3x=4y急

1/(x+y)-1/(x-y)=-3/20,2/(x+y)+3/(y-x)=-(1/5)急啊! 已知x/3=y/5=z/4,求x+y+z/y和x+y-z/y-x的值,2种方法.急等. 三元一次方程组解法 4x+y-z=12 x+y+z=26 3x+2y+z=-5 x-y=1 x-y+5z=1 2x-y+z=18 急 急、、、、、、、、、、 3(x+y)+2(x-y)=36 4(x+y)-5(x-y)=2 4(x-y)-3(x-y)=9 5(x-y)+2(x-y)=11 3(x+y)-4(x-y)=11,2(x-y)+5(x+y)=27 X:2.5=Y:3/4 求X:Y 急 先化简再求值(x-y)(x-2y)+(x-2y)(x-3y)-2(x-3y)(x-4y) x=4 y=5 x:y=3:4 y:z=2:1 x+y+z=20 急! 若2x-3y=5,则6-4x+6y=急!~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 2y-x=5,3x=4y急 如果x+2y=3 7x+5y=-1,那么(4x+8y)/3+(14x+10y)/2=?急! [(2x+y)²-y(y+4x)-8x】/4x,其中x=-1,y=2005 急 先化简,再求值：x-3(x+2y)+4(2y-x),其中x=-3,y=2 急 (x-y) (x-2y)+(x+2y) (x-3y)-2(x-3y) (x-4y)= 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 几道 数学题急啊 对了再加20分(x+y)^2-(x+y)^3(x-y)^2-(x-y)^3 (x-y)^2-(y-x)^32(x-y)-3(y-x)^24ab(a+b)^2-6a^2b(a+b)(x+y)(x-y)-(x+y)^28a(x-y)^2-4b(y-x)a=-5 a+b+c=-5.2求代数式a^2(-b-c)-3.2a(c+b)的值