(4)已知 =xln x, 则 y(10) 等于 ()-|||-(A) -dfrac (1)({x)^9} (B) dfrac (1)({x)^9} (C) dfrac (81)({x)^9} (D) -dfrac (81)({x)^9}

17、已知 D(X)=9 ,D(Y)=36 ,(rho )_(x)y=0.4 ,则 (dfrac (1)(3)X+dfrac (1)(6)Y)=D(X+Y)=-

(10)设 = (x,y)|dfrac {{x)^2}(4)+dfrac ({y)^2}(9)leqslant 1} , 则dxdy等于 () .-|||-A.

函数=dfrac (1)({3)^x-9}的定义域是（ ）=dfrac (1)({3)^x-9}=dfrac (1)({3)^x-9}=dfrac (1)(

下列选项中曲面dfrac ({x)^2}(4)+dfrac ({y)^2}(1)+dfrac ({z)^2}(9)=3上点dfrac ({x)^2}(4)+df

(B) _(x)(dfrac (y-b)(a)).-|||-(C) dfrac (1)(|a|)(f)_(x)(y-b). (D) dfrac (1)(|a|)

3、设 (x,y)=arctan dfrac (x)(y), 则 (1,1)=-|||-(A)1; (B)0; (C) dfrac {1)(2),dfrac

[题目]若 (dfrac (1)(x))=x, 则 (x)= ()-|||-A、 dfrac (1)(x)-|||-B、 dfrac (1)({x)^2}-||

(B) X=Y =dfrac (1)(2),-|||-(C) X=Y =dfrac (1)(4), (D) X=Y =0.A、AB、BC、CD、DA、AB

lim _(xarrow +infty )(({3)^x+(9)^x)}^dfrac (1{x)}= ()-|||-(A)0 (B)9 (C)9e 3 (D)∞

(B) dfrac (x)(y)((y+1))^2-|||-(C) ^2((x+dfrac {1)(x))}^2. (D) dfrac (y)(x)((y+1)