首先，你的对数写法是错的。改写 y = (1/2)ln(x²+y²)，求偏导数，得 ∂z/∂y = (1/2)[2x/(x²+y²)] = x/(x²+y²)， ∂²z/∂x∂y = (∂/∂x)[x/(x²+y²)] = [(x²+y²)-x*2x]/(x²+y²)² = ……。
