2 2 16*(x - y) - 25*(x + y)
16*(x - y)^2 - 25*(x + y)^2
/ y\ 1*(x + 9*y)*|x + -| \ 9/
(1*(x + 9*y))*(x + y/9)
-(x + 9*y)*(y + 9*x)
16.0*(x - y)^2 - 25.0*(x + y)^2
2 2 - 9*x - 9*y - 82*x*y
-9*x^2 - 9*y^2 - 82*x*y