z= xln(x+y)∂z/∂x = ln(x+y) + x/(x+y)∂^2z/∂x^2 = 1/(x+y) + [(x+y)-x]/(x+y)^2 = 1/(x+y) + y/(x+y)^2 ∂z/∂y = x/(x+y)∂^2z/∂x∂y=∂/∂y(∂z/∂x)= 1/(x+y) - x/(x+y)^2
