∂z/∂x=ln(x+y) +x/(x+y),
∂z/∂y= x/(x+y),
所以
∂²z/∂x²
=∂[ln(x+y) +x/(x+y)] / ∂x
=1/(x+y) + [(x+y) -x]/(x+y)²
∂²z/∂x∂y
=∂[ln(x+y) +x/(x+y)] / ∂y
=1/(x+y) - x /(x+y)²
∂²z/∂y²
=∂ [x/(x+y)] /∂y
= -x /(x+y)²