fx=2yx/(x^2+y^2)^2, fxx=(2y(x^2+y^2)-8x^2y)/(x^2+y^2)^3=(2y^3-6x^2y)/(x^2+y^2)^3;fy=[2(x^2+y^2)-2y^2]/(x^2+y^2)^2=2x^2/(x^2+y^2)^2, fyy=-8x^2y/(x^2+y^2)^3;
可以用对数求导法 dz/dx= Inz=2x+yIn(2x+y)z&9;*1/z=2In(2x+y)+1 z&9;=2(2x+y)^2x+y*(In(2x+y)+1)dz/dy同理可得 =(2x+y)^2x+y*(In(2x+y)+1)