5.已知x=3 y=7 而xy

5.已知x=3 y=7 而xy 已知：|x|=3,y=7,而xy 已知|x|=3,|y|=7,而xy 已知xy/x+y=3,则代数式2x-7xy+2y/-x+xy-y的值是多少? 已知x-y=3xy,且xy不等于0,则-3x+6xy+3y/(7x-5y)-9xy-(3x-y) 已知:x-y=7,xy=5,则3xy-7x-7y=? 已知x-y=5,xy=3,则3xy-7x+7y=? 已知xy为正整数,3xy=2x+3y+5.求xy=? 已知xy^2=-2求xy（2x^3y^7+5x^2y 3xy+xy+3+4xy-7化简,已知x=5,y=6 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+xy=-3,xy+y=7,试求x+2Xy+y的值 已知X+Y=6,xy=3求代数式（5XY+4X+7Y)+(6X-3XY)-(4XY-3Y)的值 已知x+y=3,xy=1,求代数式5xy+4x+7y+6x-3xy-4xy+3y的值. 已知x+y=4,xy=2,求代数式2(3x+2xy)+(2xy+3x+2y)-(4xy-7y)的值 已知,1/x+1/y=5.,求分式3x+xy+3y/x-2xy+y的值