In Problems 47-54, find the intervals on which f(x) is increasing and the intervals on which f(x) is decreasing. Then sketch the graph. Add horizontal tangent lines.
48. f(x) = 2x2 - 8x + 9
50. f(x) = x3 - 12x + 2
52. f(x) = x3 + 3x2 + 3x
54. f(x) = -x4 + 50x2