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X与X+Y一定相关. X与X+Y一定不相关. X与XY一定相关. X与XY一定不相关.
2x-2y 2xdx-2ydy f′(x2-y2)dx 2f′(x2-y2)(xdx-ydy)
∫Γf(x,y)dx. ∫Γf(x,y)dy. ∫Γf(x,y)ds. ∫Γf’x(x,y)dx+f’y(x,y)dy.
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F=x·y-xa·ya F=xa·y-ya·x F=xa·x-ya·y F=xa +ya-x-y
a与b可以是任意实数. a=b. ab=-1. ab=1.
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(xey-2y)dy+eydx=0 xy′+y=ex+y [x/(1+y)]dx-[y/(1+x)]dy=0 dy/dx=(x+y)/(x-y)
(A) ①、③. (B) ①、④. (C) ②、③. (D) ②、④.
f(x) f(x)dx f(x)+C f’(x)dx
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D(X+Y)≥DX+D Y. D(X+Y)<DX+D Y. D(X-Y)≥DX+D Y. D(X-Y)<DX+D Y.