f(x+y，xy)

=f(u，v)

=2x^2 + 3xy + 2y^2 + 6

=2(x^2 + 2xy + y^2) - xy + 6

=2(x+y)^2 - xy + 6

=2u^2 - v + 6

As you can see, if the f(u, v) function equation uses X and Y to represent two independent variables, then its expression will become:

f(x，y) = 2x^2 - y + 6