A is actually infinitesimal and can be ignored.

So when y=x,

f(x，y)=x？ +(2x？ )? =x？ +4x 4, square plus four.

Of course, it is greater than 0, that is, greater than f (0,0).

And when y=-x, f(x, y)= -x? +(2x？ )? = -x？ +4x^4=(4x？ - 1)x？

X tends to zero, 4x? -1 is of course less than 0, then f (x, y)