2x y-7)m (x y-4)=0

f(x,y)={xy/[2-√(4+xy)]=-2-√（4+xy),xy≠0；{4,xy=0,在点(0,0),(1,0)处不连续,在（1,2）处连续.再问：能简述下原因么？再答：f(0+,0+)=-4

2mxy-xy+1-2x2-4mxy+5x2+7xy=5x2-2x2-4mxy+7xy+2mxy-xy+1=3x2+（-4m+7+2m-1）xy+1，因为没有xy项，所以-4m+7+2m-1=0，解得

1.2(Xy+Xy)-3(Xy-xy)-4Xy=2*2xy-0-4xy=4xy-4xy=02.1/2ab-5aC-(3acb)+(3aC-4aC)=1/2ab-5ac-3acb-ac=1/2ab-6a

解xy互为相反数y=-x代入2y-x=-72y+y=-73y=-7所以y=-7/3x=7/3代入2x-y=4m+3,14/3+7/3=4m+34m+3=7m=1再问：某工厂有28名工人，生产螺母和螺栓

∵x2-xy=7，2xy+y2=4，∴原式=（x2-xy）+（2xy+y2）=7+4=11，故答案为：11

（x+2）^2+|y+1|=0x=-2,y=-15xy²-｛2xy²-[3xy²（4xy²-2x²y）]｝=5xy²-2xy²+3

原式=6x+4xy+2xy+3x+2y-4xy+7y=9x+9y+2xy=9(x+y)+2xy=9×4+2×2=36+4=40

|x|=4,|y|=1/2,∴x=4或x=-4,y=1/2或y=-1/2又xy

2M-7N=-1∴2（2x²y-6xy+3)-7(3xy-x²y+1)=-14x²y-12xy+6-21xy+7x²y-7=-111x²y-33xy=

由2x²-3y²=xy得(X+Y)(2X-3Y)=0所以X=-y,或者2X=3Y,当X=-y时,代入得=-5当2X=3Y,代入得=15/14

(3xy+2)^2+|7-x-y|=0,则3xy+2=0,7-x-y=0,即x+y=7,xy=-2/3(5xy+10y)-[-5x-(4xy-2y+3x)]=5xy+10y-(-5x-4xy+2y-3

（4xy+12y)+[7x-(3xy+4y-x)]=4xy+12y+7x-3xy-4y+x=xy+8x+8y=xy+8(x+y)=(-2)+8*3=-2+24=22

a-2b=(5x²y-3xy²+4xy)-2(7xy²-2xy+x²y)=5x²y-3xy²+4xy-14xy²+4xy-2x&#

首先把这个式子合并同类项,得到：3x*x+(2m-1-4n+7）xy+1由于式子中不含XY项,因此XY项前面的系数一定为0即：2m-1-4n+7=0求解得m=2n-3

(5xy+4x+7y)+(6x-3xy)-(4xy-3y)=5xy+4x+7y+6x-3xy-4xy+3y=(5xy-3xy-4xy)+(4x+6x)+(7y+3y)=-2xy+10x+10y=-2x

解x²+2y²-2xy+4y+4=0吧(x²-2xy+y²)+(y²+4y+4)=0(x-y)²+(y+2)²=0∴x-y=0,y

已知A=5x^2y-3xy^2+4xy,B=7xy^2-2xy+x^2y.（1）求A-2B的值A-2B=5x^2y-3xy^2+4xy-2*(7xy^2-2xy+x^2y)=5x^2y-3xy^2+4

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

x²-7xy+12y²=0(x-3y)(x-4y)=0x1=3yx2=4yx=3y时原式＝9y²-3y²+y²/6y²=7/6x=4y时原式

∵x-y=4xy∴原式=[2(x-y)+3xy]/[-(x-y)-2xy]=(8xy+3xy)/(-4xy-2xy)=11xy/(-6xy)=-11/6