xy>0,∂|xy|/∂x=∂(xy)/∂x=y;
xy<0,,∂|xy|/∂x=∂(-xy)/∂x=-y;
同理,
xy>0,∂|xy|/∂y=∂(xy)/∂y=x;
xy<0,,∂|xy|/∂y=∂(-xy)/∂y=-x;
∂f/∂x=(1/2)|xy|^(-1/2).∂|xy|/∂x
=±y(1/2)|xy|^(-1/2)
∂f/∂y=(1/2)|xy|^(-1/2).∂|xy|/∂y
=±x(1/2)|xy|^(-1/2)
沿y=kx方向,k>0时,xy>0
∂f/∂x=y(1/2)|xy|^(-1/2)=kx(1/2)|kx²|^(-1/2)=±(1/2)√k
∂f/∂y=x(1/2)|kx²|^(-1/2)=±(1/2)(1/√k)
沿不同方向,偏导数不同,故不存在。
追问答案是存在啊都等于0
能写出来吗这个看不太懂
追答不是0,分子分母都是无穷小,要取极限。
追问能写出来吗这个好难看懂谢谢了
追答![](https://video.ask-data.xyz/img.php?b=https://iknow-pic.cdn.bcebos.com/c75c10385343fbf231b44796bc7eca8064388fef?x-bce-process=image%2Fresize%2Cm_lfit%2Cw_600%2Ch_800%2Climit_1%2Fquality%2Cq_85%2Fformat%2Cf_auto)
![](https://video.ask-data.xyz/img.php?b=https://iknow-pic.cdn.bcebos.com/77c6a7efce1b9d1650f77ad4ffdeb48f8c546457?x-bce-process=image%2Fresize%2Cm_lfit%2Cw_600%2Ch_800%2Climit_1%2Fquality%2Cq_85%2Fformat%2Cf_auto)
追问所以这题不能用偏导数的定义算?