In #7-12, Find the critical points of the given function and classify them as local max- ima, local minima, saddle points, or none of these. 7. f(x,y) = x2 - 2xy + 3y -8y 8. f(x,y) =5+6x - x2 + xy - y' 9. f(x, y) = x2 - y? + 4x + 2y 10. f(x, y) = 400 -3x2 - Ar + 2ry - 5y + 48y 11. f(x,y) = 3 - 3x + y - 3y 12. f(x, y) = (x + y)(ry+1) 13. Let f(x, y) = 3x2 + ky2 + 9ry. Determine the values of k (if any) for which the critical point (0, 0) is : (a) A saddle point (b) A local maximum (c) A local minimum 14. Find A and B so that f(x, y) = x2 + Ar + y2 + B has a local minimum value of 20 at (1,0)