x-2y=7 xy=3 则2xy-4y+2x+5=?

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xy/x+y=3,则代数式2x-7xy+2y/-x+xy-y的值是多少? 若x-y=4,xy=1,则(2x+y-2xy)-(3x+2y+xy)-(4xy+3y-5x)= （-3x^y+2xy)-( )=4x^+xy 当x-y=2xy时,求4x-3xy-4y/xy x*x+xy=3,xy+y*y=2,则x*x+2xy+y*y=________? 已知x-y=8 xy=7,则3xy-2x+2y+4(y-x)²=? 求（3x^+y^-5xy）-【2x^-（-4xy-y^+7x^）】=? 求（3x^+y^-5xy）-【2x^-（-4xy-y^+7x^）】=? 已知x+y=4,xy=2,求代数式2(3x+2xy)+(2xy+3x+2y)-(4xy-7y)的值 x+y=8,xy=-2,求(5xy+4x+7y)+(6x-3xy)-(4xy-3y)的值 已知x/y=3/4 ,则3xy-2y/2x+4xy=? 已知x²-3xy+4x=3,y²+xy-4y+7=0,则xy-1/2(x²+y²)的值为 f(xy,x-y)=x^2+3xy+y^2,则f(x,y)=? 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 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.. 如果X-2Y=7,XY=3,则2XY-4Y+2Y+5=( ) 若x-2y=7,xy=3则2xy-4y+2y+5