已知1 x-1 y=3求5x-2xy-5y x-xy-y的值

x=1/3y=1/2x-(x+y）+（x+2y)-(x+3y)+...-(x+13y)=6y-13y=-7y=-7/2=-3.5

2x-3y=0(1)3x-y-1=0(2)由（2）得y=3x-1(3)把y=3x-1代入(1)得2x-9x+3=0∴x=3/7把x=3/7代入（3）得y=2/7∴x=3/7y=2/7

分子分母同时除以xy则分子得2（1/x+1/y）-3=2*5-3=7分母得3（1/x+1/y）+2=3*5+2=17则得7/17

1/x-1/y=3(Y-X)/XY=3Y-X=3XYX-Y=-3XY(x-2xy-y)/(5x+3xy-5y)=(-3XY-2XY)/(-15XY+3XY)=-5XY/-12XY=5/12

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)

(1)x>2→x-2>0.依基本不等式得y＝(x^2+5)/(x-2)＝[(x^2-4)+9]/(x-2)＝(x-2)+9/(x-2)+4≥2√[(x-2)·9/(x-2)]+4＝10.∴x-2＝9/

3x=8yx/y=8/3（1）x+y/y=x/y+1=8/3+1=11/3(2)2x+3y/x-2y分子分母同时除以y得=(2x/y+3)/(x/y-2)=(16/3+3)/(8/3-2)=(25/3

3x-4y/2x+y=1/26x-8y=2x+y4x=9yx/y=9/4

x+1/x=3,x^2+1/x^2+2=9x^2+1/x^2=7(x-1/x)^2=X^2+1/x^2-2=7-2=5x-1/x=±√5x^2+y^2-4x-2y+5=0(x-2)^2+(y-1)^2

(1)已知x+y=6,x²-y²=5,x^2-y^2=(x+y)(x-y)=5x-y=5/(x+y)=5/6（2）若x+y=3,x²+y²=5(x+y)^2=3

因为(x-y)/(x+y)=3,则(x＋y)/(x－y)＝1/3则5(x-y)(x+y)-(x+y)/2(x-y)＝5*3-1/(3*2)=15-1/6=89/6

x-y=1,x+y=-73（x+y)+20(x-y)-5=3×（-7）+20×1-5=-21+20-5=-6

1/x+1/y=3去分母得x+y=3xy(3x-5xy+3y)/(x-2xy+y)=[3(x+y)-5xy]/(x+y-2xy)=(9xy-5xy)/(3xy-2xy)=4xy/xy=4

1/x+1/y=(x+y)/xy=5x+y=5xy2x-3xy+2y/-x+2xy-y=[2(x+y)-3xy]/[2xy-(x+y)]=(10xy-3xy)/(2x-5xy)=7xy/(-3xy)=

已知x²+y²+5=2x+4y所以(x-1)²+(y-2)²=0故x=1,y=2所以（2x²-（x+y)(x-y))×((x+y-1)(x-y+1)+

4x=9yx=9/4*y(1)(x+y)/y=[(9/4)y+y]/y=(9/4+1)y/y=9/4+1=13/4(2)(y-x)/2x=[y-(9/4)y]/[2*(9/4)y]=(1-9/4)y/

原式=(x-y)²-3(x-y)+1=3²-3×3+1x-y用3代替=1再问：3X+3Y是怎么转化成这个的3(X-Y)，什么理由再答：是-3x+3y不是3x+3y

题目是否抄错?应该是（2X-3XY+2Y）/（X+2XY+Y）吧?1/X+1/Y=5（Y+X）/XY=5X+Y=5XY(2X-3XY+2Y)/(X+2XY+Y)=〔2（X+Y）-3XY〕/（X+Y+2

1,-3再问：过程。。。再答：★（x²－2x)+（y²－4y）=5★（x－1）²+（y－2）²=1+4－5★（x－l）²=0,（y－2）²=

原式=x-x+x-x+……-x+(2-1+4-3+5-4+……+2008-2007-2009）y=0+(1×1004-2009)y=-1005y=1005/2