find all the critical points and determi Whether each is a local maximum, local minimum, a saddle point, or none of these. 15. f(x, y) = x2 + 4x + y2 16. f(x, y) = x2 + xy + 3y 17. f(x, y) = x2 + y2 + 6x - 10y + 8 18. f(x, y) = y3 – 3xy + 6x 19. f(x, y) = x3 + y2 – 3x2 + 10y + 6 20. f(x, y) = x + y3 – 6y2 – 3x +9 21. f(x, y) = x + y2 – 3x2 – 3y + 10 22. f(x, y) = x2 – 2xy + 3y2 – 8y 23. f(x, y) = x3 – 3x + y2 – 3y 24. f(x, y) = 400 - 3x2 - 4x + 2xy - 5y2 + 48 y