If f(x, y) = {x|x|√(x2+y2) , x ≠ 00 , x = 0 then fx(0, 0) + fy(0, 0) is equal to:

If f: ℝ2 → ℝ is a function given by f(x,y) = (x3 - y3)/(x3 + y3) for (x,y) ≠ (0,0) and f(0,0) = 1, then which of the following options is correct?

Find the critical points of the function f(x, y) = [sin-1(y2) * (y2 + 3y) * sin(y6 + 7y)] / (y9 + y10) + 10x.