9.设z=e^xy-cos e^xy，则dz=（）.（A）e^xy(1-xsin e^xy)(ydx-dy) （B）e^xy(1+sin e^xy)(ydx+xdy)（C）xe^xy(1-ysin e^xy)(dx+dy) （D）xe^xy(1+sin e^xy)(dx-dy)

E(XY)=1.A. X的边缘密度函数为$f_{X}(x)=\begin{cases}e^{-x},x>0,\\0,其他;\end{cases}$B. $ρ_{

则 A= __ ___, E(XY)=-|||-__

设 =(e)^xy+ln ((x)^2+(y)^2) 则（ ）A =(e)^xy+ln ((x)^2+(y)^2)B =(e)^xy+ln ((x)^2+(y)

E(XY)=E(X)E(Y)

由方程^2=(e)^xy+cot (x)^2确定的函数^2=(e)^xy+cot (x)^2的导数^2=(e)^xy+cot (x)^2.A.^2=(e)^xy

设=4, =1, rho xy=0.5，则=4, =1, rho xy=0.5( ) A.=4, =1, rho xy=0.5B.=4, =1, rho xy=

若X,Y相互独立，E(X)=a,E(Y)=b，则E(XY)= ()。A. 1B. 2C. 3D. -1

5.设 =arctan (xy), 而 =(e)^x, 求 dfrac (dz)(dx).

设(x,y)=sin xy,则(x,y)=sin xy设,则

求曲面=(e)^z-2xy+3在点=(e)^z-2xy+3处的切平面方程及法线方程。求曲面在点处的切平面方程及法线方程。