7.设 (x,y)=dfrac (1)(xy),r=sqrt ({x)^2+(y)^2} _(1)= (x,y)|(x,y)in {R)^2 dfrac (1)(k)xleqslant yleqslant kx gt 1 为常数},-|||-_(2)= (x,y)|(x,y)in {R)^2,xgt 0,ygt 0} -|||-(1) lim f(x,y)是否存在?为什么?-|||- x,y in D-|||-(2) lim f(x,y)是否存在?为什么?-|||-(x,y)in (D)_(2)

(9)设 f(x,y)= dfrac (1)({({x)^2+(y)^2)}^2},1leqslant xleqslant 3, dfrac (sqrt {3)

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

设 (x,y)=dfrac ({x)^2+(y)^2}({e)^xy+xysqrt ({x)^2+(y)^2}} ,则 (f)_(x)(1,0)= __ _.

设 y = y ( x ) 满足 +dfrac (1)(2sqrt {x)}y=2+sqrt (x), y(1)=3,求 y = y ( x ) 的渐近 线设y

1.设 sin y+(e)^x-x(y)^2=0, 求 dfrac (dy)(dx).-|||-2.设 ln sqrt ({x)^2+(y)^2}=arctan

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

7.设 =arctan dfrac (1)(x) ,则 dfrac ({d)^2y}(d{x)^2}(x)_(x=1)= __

求下列函数的自然定义域：(1)y=sqrt(3x+2)；(2)y=dfrac(1)(1-{x)^2}；(3)y=dfrac(1)(x)-sqrt(1-(x)^2

7.试求下列极限:-|||-(1) lim _((x,y)arrow +infty )dfrac ({x)^2+(y)^2}({x)^4+(y)^4}-|||-

7.设 =dfrac (y)(f({x)^2-(y)^2)} 其中f为可导函数,验证: dfrac (1)(x)dfrac (partial z)(partia