
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+
若X,Y相互独立,E(X)=a,E(Y)=b,则E(XY)= ()。A. 1B. 2C. 3D. -1
设 =(e)^xy+ln ((x)^2+(y)^2) 则( )A =(e)^xy+ln ((x)^2+(y)^2)B =(e)^xy+ln ((x)^2+(y)
对于随机变量X、Y,若E(XY)=E(X)·E(Y),则()。A. D(XY)=D(X)·D(Y)B. D(X+Y)=D(X)+D(Y)C. D(X-Y)=D(
设X,Y为随机变量,若E(XY)=E(X)E(Y),则( ).A. X,Y独立B. X,Y不独立C. X,Y相关D. X,Y不相关
二维随机变量(X,Y)满足E(XY)=E(X)E(Y),则().A. D(XY)=D(X)D(Y)B. D(X+Y)=D(X-Y)C. X与Y独立D. X与Y不
E(XY)=E(X)E(Y)
(单选题,3分) 若X,Y相互独立,E X=a,E Y=b,则E(XY)=()A. 1B. 2C. 3D. -1
已知E(X)=1,E(Y)=2,E(XY)=3,则X,Y的协方差Cov(X,Y)=( )A. 2B. 1C. 0D. 3
已知函数 (x,y)=(e)^xy+y ,则 gradf(0,1)= __