Find all values of x and y such that both fr = 0 and fy = 0 a) f (x, y ) = 622 + 6y2 + 6xy + 36x -5 b) f (x, y) = 9xy - 23-13 -6 3. Find all second order partial derivatives evaluated at the point (1, -1) for the following: a ) f ( 1 , y ) = 41242 - 16*2 + 4y 1 ) f (1, y ) = 2:12 + y4 - 16xy-10