z=arctan(y/x)+ln√(x²+y²)=arctan(y/x)+½ln(x²+y²)
∂z/∂x=(y/x)'/[1+(y/x)²]+½·2x/(x²+y²)
=-y/(x²+y²)+x/(x²+y²)
=(x-y)/(x²+y²)
∂z/∂y=(y/x)'/[1+(y/x)²]+½·2y/(x²+y²)
=x/(x²+y²)+y/(x²+y²)
=(x+y)/(x²+y²)
