6. Find and classify all the critical points of the following functions: a) f(X, Y) = 10 - X2 - Y2 b) f (X, Y) = 12X2 + Y3 - 12XY c) f (X, Y ) = X3 + Y3 - 3XY + 4 d) f(X, Y) = 3X2Y + 13 - 3X2 - 342+2