
设x^2+2xy-y^2=a^2,求dy/dx及d2y/dx2
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x^2+2xy-y^2=a^2
微分得2xdx+2ydx+2xdy-2ydy=0,
(x+y)dx=(y-x)dy,
所以y'=dy/dx=(x+y)/(y-x),
d2y/dx2
=(1+y')/(y-x)-(x+y)(y'-1)/(y-x)^2
=2y/(y-x)^2-2x(x+y)/(y-x)^3
=(2y^2-4xy-2x^2)/(y-x)^3.
微分得2xdx+2ydx+2xdy-2ydy=0,
(x+y)dx=(y-x)dy,
所以y'=dy/dx=(x+y)/(y-x),
d2y/dx2
=(1+y')/(y-x)-(x+y)(y'-1)/(y-x)^2
=2y/(y-x)^2-2x(x+y)/(y-x)^3
=(2y^2-4xy-2x^2)/(y-x)^3.
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