3x^2-(5y^2+6xy)+（7y^2-3x^2)其中x=-2分之1,y^2=3分之1.先化简,再求值

1、(-7x^y)(2x^y-3xy^3+xy) 2、((x-y)^6)/((y-x)^3)/(x-y) 因式分解 x-xy+3y-3x 2x+xy-y-4x+5y-6 3(x-y)²-7(x-y)+8(x-y)²+6(x-y)求详细过程（2） 5xy-4x²y-8xy²+3xy-xy²+4x²y x^3y(-4y)^2+(-7xy)^2*(-xy)-5xy^3*(-3x)^2 5x^3y*(-3y)^2+(-6xy)^2*(-xy)+xy^3*(-4x^2) x+y=8,xy=-2,求(5xy+4x+7y)+(6x-3xy)-(4xy-3y)的值 x(y-x)-y(x-y) (-3xy)*5c^2Y+6x^2(7/2xy^2-2y^2) 诺x+y=6,xy=4 求(5x-4y-3xy)-(8x-y+2xy) 已知xy^2=-6,求-xy(x^3y^7-3x^2y^5-y）的值 x^2-xy-6y^2+3xy+y+2 已知x的相反数为y-6,y的倒数为2x,求出代数式(5xy+4x+7y)+3(2x-xy)-(4xy-3y)的值 已知x的相反数为y-6,y的倒数为2x,求出代数式(5xy+4x+7y)+3(2x-xy)-(4xy-3y)的值 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 4xy-[(x²+5xy-y²)-(3xy-1/2y)]x=3,y=-6 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 求（3x^+y^-5xy）-【2x^-（-4xy-y^+7x^）】=? 求（3x^+y^-5xy）-【2x^-（-4xy-y^+7x^）】=? 化简：x-y/x+3y÷x^-y^2/x^2+6xy+9y^2-(xy/x+y) 将x²+2xy+y²+3x+3y+2和2x²-32xy+y²+7x-5y+6因式分解