4.讨论下列函数在点(0,0)处的极限是否存在.-|||-(1) =dfrac (xy)({x)^2+(y)^4};-|||-(2) =dfrac (x+y)(x-y).

4.证明:(1) lim _((x,y)arrow (0,0))dfrac (x+y)(x-y) 不存在;(2)-|||-lim _((x,y)arrow (0

函数f(x,y)= dfrac (xy)({x)^2+(y)^2},(x)^2+(y)^2neq 0-|||-0, ^2+(y)^2=0在点（0，0）处（）。

4.函数f(x,y)=x^2+xy+y^2在点(0,0)处（）.A. 有极大值;B. 有极小值;C. 无极值;D. 点(0,0)不是驻点.

6.讨论下列函数的连续性:-|||-(1) (x,y)=dfrac ({x)^2-(y)^2}({x)^2+(y)^2}-|||-(2) (x,y)=dfrac

有关函数f(x,y)= dfrac (xy)(sqrt {{x)^2+(y)^2}},(x)^2+(y)^2neq 0-|||-)在点（0，0）的性态，下列说法

试求下列极限(包括非正常极限):-|||-(1) lim _((x,y))(dfrac ({x)^2(y)^2}({x)^2+(y)^2}-|||-(2) li

6.求下列各极限:-|||-(1) lim _((x,y)arrow (0,1))dfrac (1-xy)({x)^2+(y)^2} :-|||-(3) lim

6.求下列各极限:-|||-(1) lim _((x,y)arrow (0,1))dfrac (1-xy)({x)^2+(y)^2} ;-|||-(3) lim

1.试求下列极限(包括非正常极限):-|||-(1) lim _((x,y))(0,0)dfrac ({x)^2(y)^2}({x)^2+(y)^2} ;-||

4.求 =dfrac (x-y)(x+y) 的通解.