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 (x-y)(x+y);-|||-(3) (x,y)= ^2+{y)^2}},(x)^2+(y)^2neq 0 0,(x)^2+(y)^2=0 .

1.讨论下列函数的连续性:-|||-(1) (x,y)=tan ((x)^2+(y)^2) ;-|||-(2) (x,y)=[ x+y] ;-|||-(3)

设 (x+y,x-y)=2((x)^2+(y)^2)(e)^(x^2-{y)^2}, 则 _(x)(x,y)-(f)_(y)(x,y)= __

6.求下列各函数的函数值:-|||-(1) (x,y)=([ dfrac {arctan (x+y))(arctan (x-y))] }^2 ,求 (dfrac

5.设函数 =dfrac (x+y)(x-y), 则 dz=-|||-(A) dfrac (2(xdy-ydx))({(x-y))^2} (B) dfrac (

2.已知 dfrac (x)(x+y)=dfrac (1)(3) ,求 dfrac ({x)^2-(y)^2}(2xy+{y)^2} 的值.

6.设 (x,y)=dfrac (x-{y)^2+(y)^3}(2x+{y)^2}, 则,lim h(x,y)等于 ()-|||-(A) dfrac (1)(2

计算 =(int )_(L)dfrac ((x-y)dx+(x+y)dy)({x)^2+(y)^2}-|||-f[(x-y)(x+(x+y)dy)/(x^2+x

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)(

设=dfrac (2x)({x)^2-(y)^2} ，则 =dfrac (2x)({x)^2-(y)^2}=dfrac (2x)({x)^2-(y)^2}___

设函数 (x,y)=1-dfrac (cos sqrt {{x)^2+(y)^2}}(tan ({x)^2+(y)^2)} ,则当定设函数 (x,y)=1-df