已知X+Y=5500 求 (XY+2000X)/4000的最大值?当X+Y变化时,X和Y之间比值是否变化?

已知x-xy=8,xy-y=-9,求x+y-2xy的值 已知xy^2=-2,求-xy（x^2y^5-xy^3-y)的值. 已知xy^2=-2 求-xy(x^2y^5-xy^3-y) 已知xy/x+y=3,求代数式3x-5xy+3y/-x+3xy-y的值.已知xy/x+y=3,求代数式3x-5xy+3y/-x+3xy-y的值. 已知X+Y=3XY,求2X+2Y-XY除以X+Y+2XY的值,要算理.已知X+Y=3XY,求2X+2Y-XY除以X+Y+2XY的值,要算理. 已知x:y=3:2,求(x²+3xy)/(xy+2xy) 已知X+y=6,xy=7 求x^2y+xy^2=已知X+y=6,xy=7 x^2y+xy^2 已知x+y=12,xy=9,求(x+3xy+y)/(xy+xy)的值 已知x^2-xy=5,xy-y^2=-3求3(x^2-xy)-xy+y^2 已知x^-xy=5,xy-y^=-3,求式子3(x^-xy)-xy+y^的值 已知：x+xy=14,y+xy+x=28.求x+y的值. 已知:x/y=-2,求x^2-2xy-3y^2/x^-6xy 已知:x-xy=40,xy-y=-20,求代数式x-y和x+y-2xy的值. 已知x-xy =21,xy-y=-12,分别求式子x-y与x-2xy +y 已知,xy/x+y=3,求代数式3x-5xy+3y/-x+3xy-y的值 已知y=3xy+x,求代数式2x+3xy-2y/x-2xy-y的值 已知x+y=3xy,求2x+2y-xy/x+y+2xy的值、 已知xy/x+y=3,求代数式2x-5xy+2y/x-3xy+y