2(x-y)-3(1/3x-2y)+5,其中x=1999,y=-1/4.（先化简,再求值）要清楚的写出过程急

(x-y)/(x+y)=3求( 3x-2y-1)/(x+y-5) {(x+y)/2+(x-y)/3=1,(x+y)-5(x-y)=2.求x=?,y=? 化简:5(x-y)+2(x+y)-3(x-y) 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 (2x-3y)-(5x-y) (2X-3y)-(5x-y) (2x-5y)(3x+y) 因式分解(x-2y)(x-3y)+(x-4y)(x-5y)+（x-3y)(x-5y)+(x-2y)(x-4y) 绝对值化简|x-y+1|-2|y-x-3|+|y-x|+5y (2x+y)²+(x+3y)(x-3y)-x(5x+8y)因式分解 x²/xy-x/y 3x/4x+y - x-2y/4x+y (1-y/y+x)÷x/y²-x² [2(x-y)^5-8(x-y)^2(x+y)+6y(x-y)^2]÷2(x-y)^2[2(x-y)^3-8(x-y)^2(x+y)+6y(x-y)^2]÷2(x-y)^2 x(x-y)2-(y-x)3 （2x+y-1)(2x-y+1)-(3x+y)(3x-y),其中x=5分之1, (x+2y)(x-2y)-(2x-y)的平方+(3x-y)(2x-5y)其中x=-1 y=13分之1 x+y=1 5x-2y=8 3x-y=7 2x-y=3x表示y和用y表示x 若2/x-1/y=3,求[y/x-y/x-y(x-y/x-x+y)]/x-2y/x的值