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以y为例，要加个|2xy/(x^2+y^2)|<=1(他用的是|xy/(x^2+y^2)|<=1)
df/dy=x*[2xy/(x^2+y^2)]-x*[2xy/(x^2+y^2)][y^2/(x^2+y^2)]
|df/dy|<=|x*[2xy/(x^2+y^2)]|+|-x*[2xy/(x^2+y^2)][y^2/(x^2+y^2)]|
=|x|*|[2xy/(x^2+y^2)]|+|-x|*|[2xy/(x^2+y^2)|*|y^2/(x^2+y^2)|<=|x|+|x|=2|x|
df/dy=x*[2xy/(x^2+y^2)]-x*[2xy/(x^2+y^2)][y^2/(x^2+y^2)]
|df/dy|<=|x*[2xy/(x^2+y^2)]|+|-x*[2xy/(x^2+y^2)][y^2/(x^2+y^2)]|
=|x|*|[2xy/(x^2+y^2)]|+|-x|*|[2xy/(x^2+y^2)|*|y^2/(x^2+y^2)|<=|x|+|x|=2|x|
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