[单选题]将2(x+y)-3(x-y)-4(x+y)+5(x-y)-3(x-y)合并同类项得()。A.-3x-yB.-2(x+y)C.-x+yD.-2(x+y)
[单选题]将2(x+y)-3(x-y)-4(x+y)+5(x-y)-3(x-y)合并同类项得()。A.-3x-yB.-2(x+y)C.-x+yD.-2(x+y)
4.讨论下列函数在点(0,0)处的极限是否存在.-|||-(1) =dfrac (xy)({x)^2+(y)^4};-|||-(2) =dfrac (x+y)(
(4)求微分方程的通解.-|||-① y=xy ;-|||-③ dfrac (dy)(dx)=dfrac (y)(x)+(cos )^2dfrac (y)(x)
5.设函数 =dfrac (x+y)(x-y), 则 dz=-|||-(A) dfrac (2(xdy-ydx))({(x-y))^2} (B) dfrac (
4.设随机变量(X,Y)满足D(X+Y)=D(X-Y),则必有()A. X与Y相互独立B. X与Y不相关C. X与Y不相互独立D. D(X)=0或D(Y)=0
(4)齐次方程 dfrac (dy)(dx)=-dfrac (4x+3y)(x+y) 的通解是 ()-|||-(A) ln (y+2x)+dfrac (x)
4.证明:(1) lim _((x,y)arrow (0,0))dfrac (x+y)(x-y) 不存在;(2)-|||-lim _((x,y)arrow (0
6.求下列各函数的函数值:-|||-(1) (x,y)=([ dfrac {arctan (x+y))(arctan (x-y))] }^2 ,求 (dfrac
设 (x+y,x-y)=2((x)^2+(y)^2)(e)^(x^2-{y)^2}, 则 _(x)(x,y)-(f)_(y)(x,y)= __