3xy-7/4xy^2-7x^4,-3x+1/7,x^m+1 -2x^m +3^m+2项是多少?并指出它们是几次几项式.

x^3y(-4y)^2+(-7xy)^2*(-xy)-5xy^3*(-3x)^2 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^）】=? xy-2{xy-[3x^2y-(4xy^2+0.5xy)]+4x^2y}+6xy^2 已知x+y=4,xy=2,求代数式2(3x+2xy)+(2xy+3x+2y)-(4xy-7y)的值 （X-3xy+4xy-y）-[-2x-2（xy-3/2xy）-y]+6xy的过程 x+y=8,xy=-2,求(5xy+4x+7y)+(6x-3xy)-(4xy-3y)的值 7x^2y-[xy+4(xy^2+1/8xy)-3x^2y]答对给分 -3(2x^2-xy)+4(x^2+xy-6) （-3x^y+2xy)-( )=4x^+xy 4(x²+xy-6)-3(2x²-xy) -3(2x²-xy)+4(x²+xy-6) (-3xy+2xy)-(4xy+xy) 合并同类项：(3x^2-2xy+7)-(-4X^2+5xy+6) -3（2x的平方+xy）+4(2x的平方-xy-7),合并同类项. 已知1/x - 1/y = 2,求3x+7xy-3y / 2x-3xy-2y的值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)=-1/71/x-1/y=2到(y-x)/xy=2 是怎么变化的.. 化简：3x^2-[5xy-(x^y-3xy)+4x^2y-8xy]