3xy-7x+7y=3xy-7（x-y） =3×3-7×5 =-26．3xy-7（x-y）谁告诉我为什么这么做?

（ ）+x^2-xy+2y^2=2x^2+xy+3y^23x^2+xy-7y^2-( )=x^2-2xy+y^2 求（3x^+y^-5xy）-【2x^-（-4xy-y^+7x^）】=? 求（3x^+y^-5xy）-【2x^-（-4xy-y^+7x^）】=? x-y=3,xy=5,求代数式（7xy+4x-7y）+(6x-3xy)-(3y+4xy)的值快 已知xy/x+y=3,则代数式2x-7xy+2y/-x+xy-y的值是多少? 已知1/x-1/y=2,求3x+7xy-3y/2x-3xy-2y答案是1/x-1/y=2(y-x)/xy=2 得：x-y=-2xy(3x+7xy-3y)/(2x-3xy-2y)=[3(x-y)+7xy]/[2(x-y)-3xy]=(-6xy+7xy)/(-4xy-3xy)=xy/(-7xy)=-1/7 谁能帮我解释一下=[3(x-y)+7xy]/[2(x-y)-3xy]=(-6xy+7xy)/(-4xy-3xy) 是怎 已知x-y=3xy,且xy不等于0,则-3x+6xy+3y/(7x-5y)-9xy-(3x-y) x^3y(-4y)^2+(-7xy)^2*(-xy)-5xy^3*(-3x)^2 化简求值x-4y/2x-7xy+3y×x-2xy-3y/2x+3xy-2y÷3x-6y/4x-4xy+y,其中：x=1999,y=-1 1、(-7x^y)(2x^y-3xy^3+xy) 2、((x-y)^6)/((y-x)^3)/(x-y) 已知x=3y,试求（x-y)^2+2xy-y^2/x^2+7xy+y^2 已知x=3y,试求(x-y)^2+2xy-y^2/x^2+7xy+y^2 已知xy^2=-6,求-xy(x^3y^7-3x^2y^5-y）的值 已知:x-y=7,xy=5,则3xy-7x-7y=? 已知x-y=5,xy=3,则3xy-7x+7y=? x-y=5,xy=3,则3xy-7x+7y= 若x+y=8,xy=7,球3xy-x-y+4的值 x加y=8,xy=7,求3xy减x减y+4