已知x²-5x y-1=0,则x y的最大值为

1/x-1/y=(y-x)/xy=-3y=x=-3xyx-y=3xy所以原式=[5(x-y)+xy]/(x-y)-xy]=[5(3xy)+xy]/[(3xy)-xy]=16xy/2xy=8

13x^2-6xy+y^2-4x+1=09x²-6xy+y²+4x²-4x+1=0(3x-y)²+(2x-1)²=0∵(3x-y)²≥0&n

（3x-4y+2xy）-（2x-5y+5xy）=x+y-5xy=10

由x+y-3xy+5=0得x+y+5=3xy．∴2xy+5≤x+y+5=3xy．∴3xy-2xy-5≥0，∴（xy+1）（3xy-5）≥0，∴xy≥53，即xy≥259，等号成立的条件是x=y．此时x

楼主是不是把题写错了呀,应该是（X+2)^2+/Y+1/=0吧因为（X+2)^2大于等于0/Y+1/大于等于0所以只有同时为0时,和才为0即X=-2Y=-1带入原式=4

（x+2）平方+|y+1|=0则x+2=0,y+1=0所以x=-2.y=-1xy平方=-2.x平方y=-45xy*-｛2x*y-【3xy*-（4xy*-2x*y）】｝=5*（-2）-【2*（-4）-3

根据已知得到x=-2y=-1原式=5xy^2-{2x^2y-[3xy^2-(4xy^2-2x^2y)]}=5xy^2-{2x^2y-[3xy^2-4xy^2+2x^2y]}=5xy^2-{2x^2y-

1/x-1/y=3(y-x)/xy=3y-x=3xyx-y=-3xy(5x+xy-5y)/(x-xy-y)=[5(x-y)+xy][(x-y)-xy]=(-15xy+xy)/(-3xy-xy)=-14

答：x+y=-1,xy=-2-5（x+y)+(x-y)+x(xy+y)=-5x-5y+x-y+xy(x+1)=-4x-6y+(-2)(x+1)=-4x-6y-2x-2=-6x-6y-2=-6(x+y)

x^2y+xy^2=xy(x+y)=1/5

解(x+1)平方+/y-1/=0∴x+1=0,y-1=0∴x=-1,y=1∴2(xy-5xy平方)-(3xy平方-xy)=(2xy+xy)+(-10xy平方-3xy平方)=3xy-13xy平方=3×(

第一题：加入一项,很简单哦.很好想,有助于你的头脑边聪明哦.第二题：插入（1-1/2）最后再除以1/2

设1/x-1/y=t(5x+xy-5y)/(x-xy-y)=(1-5(1/x-1/y))/(-1-(1/x-1/y))=(5t-1)/(t+1)至于是1/x-1/y=3还是1/y-1/x=3都差不多

xy+1/xy>=2√（xy*1/xy）=2(当xy=1/xy即xy=1时取等号)x/y+y/x>=2√（x/y*y/x）=2(当x/y=y/x即x=y取等号)当x=y=1时可以同时满足两项的等号要求

|x+2|+(y-1/2)的平方=0所以x+2=0y-1/2=0所以x=-2,y=1/2所以原式=4xy-x²-5xy+y²+x²+3xy=2xy+y²=-2+

因为xy/（x+y）=1/2所以x+y=2xy原式=3（x+y)-5xy/(-x-y+3xy)=3*2xy-5xy/(-2xy+3xy)=xy/xy=1

已知x+y=5,xy=3,x³+y³=(x+y)(x²-xy+y²),则1/x³+1/y³=?解,得：因为x+y=5所以(x+y)^2==2

x²-3xy+4x=3,y²+xy-4y+7=0相加得x²-2xy+y²+4（x-y）+4=0（x-y）²+4（x-y）+4=0（x-y+2）=0,得

13x^2-6xy+y^2-4x+1=09x^2-6xy+y^2+4x^2-4x+1=0(3x-y)^2+(2x-1)^2=03x-y=0,2x-1=0,x=0.5y=3*0.5=1.5(xy-x^2

因为,x-y=3xy所以：-3x+6xy+3y/[(7x-5y)-9xy-(3x-y)]=[-3(x-y)+6xy]/[(7x-5y)-9xy-(3x-y)]=[-3*3xy+6xy]/[7x-5y-