Solution: z=ln[y(x+y)]=lny+ln(x+y)
z/? x = 1/(x+y);
z/? y =( 1/y)+ 1/(x+y);
z/? x? =- 1/(x+y)? ;
z/? y? =- 1/y? - 1/(x+y)? ;
z/? x? y=- 1/(x+y)? .