z=xln(x+y)∂z/∂x=x/(x+y) + ln(x+y)∂z/∂y=x/(x+y)∂²z/∂x²=(x+y-x)/(x+y)² +1/(x+y)=(y+x+y)/(x+y)²=(x+2y)/(x+y)²∂²z/∂y²=-x/(x+y)²∂²z/∂x∂y=-x/(x+y)² +1/(x+y)=(-x+x+y)/(x+y)²=y/(x+y)²
