求z=x*2+y*2-xy+x+y在D：x小于等于0,y小于等于0,x+y大于等于-3上的最大值和最小值.用拉格朗日乘子法

求函数z=x^2-xy+y^2在区域|x|+|y| 求函数z=x^2-xy+y^2在区域|x|+|y| 若x+y=6,xy=-8,求代数式(x+y-z)^2+(x+y-z)(x+y+z)-2·z(x-y) 若x-y=6,xy=-8,求代数式(x+y+z)²+(x-y-z)(x-y+z)-2·z(x+y)的值 已知XY=2(X+Y),YZ=4(Y+Z),ZX=5(Z+X),求X,Y,Z已知XY=2(X+Y),YZ=4(Y+Z),ZX=5(Z+X),求X,Y,Z 方程组xy/x+y=2 yz/y+z=4 zx/x+z=5 求x,y,z 已知实数x,y,z,满足那么x+y=6,z^2=xy-9,求(x+y)^z 若|x-3|+|y+2|+|2z+1|=0,求(xy-yz)(y-x+z) 已知方程组x+2y-z=0,2x-y+8z=0,求x*x+y*y+z*z/xy+yz的值 设z=(x+2y)e^xy,求dz 已知4x-3y=6z 2x+4y=14z 求2xy/x^+y^ 已知4x-3y=6z 2x+4y=14z 求2xy/x^+y^ 分式题:xy=x+y,yz=2(y+z),zx=3(z+x),求xyz/(xy+yz+xz)xy=x+y,yz=2(y+z),zx=3(z+x),求xyz/(xy+yz+xz) 已知3x－y+2z=x+2y+3z=0,求（3x^－xy+2y^)/(2x^+4xy+y^)值? 设z=f(xy^2-x^2y)求z对xy的二阶偏导 已知XY+X/X+Y+1=2 XZ+2X/X+Z+2=3 (Y+1)(Z+2)/Y+Z+3=4 求X+Y+Z的值已知(XY+X)/(X+Y+1)=2 (XZ+2X)/(X+Z+2)=3 [(Y+1)(Z+2)]/(Y+Z+3)=4 求X+Y+Z的值 若x+y=4,x^3+y^3=(x+y)(x^2-xy+y^2)z,求x^3+12xy+y^3的值. z=f(x^2-y^2,xy),求z关于y的偏导