计算dxdy,其中f（u)具有连续的导数,（s)为锥面与两球面x²+y²十z²=1,x²+

A . 半正定 B . 正定 C . 半负定 D . 负定

A . A B . B C . C D . D

A . 2f’（x2+y2） B . 4x2f"（x2+y2） C . 2’（x2+y2）+4x2f"（x2+y2） D . 2xf"（x2+y2）

A .https://assets.asklib.com/psource/2015102916175366158.jpg B .https://assets.asklib.com/psource/2015102916180562443.jpg C .https://assets.asklib.com/psource/2015102916182269156.jpg D .https://assets.asklib.com/psource/2015102916183448861.jpg

设<img src='https://img2.soutiyun.com/ask/2021-01-10/979126072378366.png' />其中f(u,v)具有二阶连续偏导数,g(u)具有二阶导数,则<img src='https://img2.soutiyun.com/ask/2021-01-10/979126085741131.png' />=().

设<img src='https://img2.soutiyun.com/ask/2021-01-29/98076540372081.png' />，其中f(t)具有连续导数，且<img src='https://img2.soutiyun.com/ask/2021-01-29/980765415471482.png' />。

设f(x)具有一阶连续导数，F(x)=f(x)(1+|sinx|)，则f(0)=0是F&39;(0)存在的( )． (A) 必要但非充分的条件 (B) 充分但非必要的条件。 (C) 充分必要条件 (D) 既非充分也非必要条件

求函数z=f(u，x，y)，u=xe^y的<img src='https://img2.soutiyun.com/ask/2020-09-25/969881639212366.png' />，其中f具有二阶连续偏导数。

若f(x)在点x₀具有直到n阶连续导数，并且<img src='https://img2.soutiyun.com/ask/2021-01-22/980164129217826.png' />那么当n为奇数时，f(x₀)非极值：当n为偶数而f⁽ⁿ⁾(x₀)＞0时，f(x₀)为极小值：当n为偶数而f⁽ⁿ⁾(x₀)＜0时，f(x₀)为极小大值.

设F(x+y+z，x²+y²+z²)=0，F对各变量具有一阶连续偏导数，求由F=0所确定的函数z=f(x，y)的梯度．

设<img src='https://img2.soutiyun.com/ask/2020-08-07/965660038068891.png' />其中f的偏导数连续,求<img src='https://img2.soutiyun.com/ask/2020-08-07/965660050357594.png' />

求下列函数的二阶偏导数(其中f具有二阶连续偏导数): <img src='https://img2.soutiyun.com/ask/2021-01-02/978463370161912.png' />

设函数f(u,v)在R²上具有二阶连续偏导数。证明:函数 <img src='https://img2.soutiyun.com/ask/2021-02-02/981107588858527.png' />

设函数f(x,y)在(x0，y0)的某邻域内具有连续二阶偏导数，且<img src='https://img2.soutiyun.com/ask/2019-07-12/931776851413807.png' />，则<img src='https://img2.soutiyun.com/ask/2019-07-12/931776867607056.png' />()。 A.必为f(x,y)的极小值 B.必为f(x,y)的极大值 C.必为f(x,y)的极值 D.不一定是f(x,y)的极值

设函数u(x,y,z)和v(x,y,z)在闭区域Ω上具有一阶及二阶连续偏导数，证明 <img src='https://img2.soutiyun.com/ask/2020-12-21/977438271832634.png' /> 其中<img src='https://img2.soutiyun.com/ask/2020-12-21/977437317641057.png' />是闭区域Ω的整个边界曲面，<img src='https://img2.soutiyun.com/ask/2020-12-21/977438285982443.png' />为函数v(x,y,z)沿<img src='https://img2.soutiyun.com/ask/2020-12-21/977437317641057.png' />的外法线方向的方向导数。这个公式叫做格林第一公式.

设<img src='https://img2.soutiyun.com/ask/2020-11-02/973192351945649.png' />,其中f具有连续偏导数,g可导,求<img src='https://img2.soutiyun.com/ask/2020-11-02/973192369901676.png' />.

试用直接代入法证明,以<img src='https://img2.soutiyun.com/ask/2020-07-03/962616698685261.png' />或<img src='https://img2.soutiyun.com/ask/2020-07-03/962616708606829.png' />为变量的具有二阶连续偏导数的任意函数u,均是齐次波动方程<img src='https://img2.soutiyun.com/ask/2020-07-03/962616719570456.png' />的解。

设f(x)为<img src='https://img2.soutiyun.com/ask/2021-02-02/981112914240121.png' />上以2π为周期，且具有二阶连续导数的函数, 记 <img src='https://img2.soutiyun.com/ask/2021-02-02/981112923970678.png' /> <img src='https://img2.soutiyun.com/ask/2021-02-02/981112932118144.png' />

设f(t)在区间(a,b)上具有连续导数，<img src='https://img2.soutiyun.com/ask/2021-01-28/980701134945681.png' />.定义D上的函数。 <img src='https://img2.soutiyun.com/ask/2021-01-28/980701153716755.png' /> <img src='https://img2.soutiyun.com/ask/2021-01-28/980701165529431.png' />

设f(x,y)具有连续偏导数,且满足<img src='https://img2.soutiyun.com/ask/2021-01-10/979124316503936.png' />求<img src='https://img2.soutiyun.com/ask/2021-01-10/979124324709406.png' />.

.设函数u=f(x,y,z)有连续偏导数，且<img src='https://img2.soutiyun.com/ask/2021-01-02/978463716012693.png' />是由<img src='https://img2.soutiyun.com/ask/2021-01-02/978463730334512.png' />所确定的隐函数，求du.