In Problem 79 - 86, find the critical values, the intervals on which f (x) is increasing, the intervals on which f (x) is decreasing, and the local extrema. Do not graph.
80. f(x) = 9/x + x
82. f(x) = 3 - 4/x - 2/x2
84.f(x) = x2/x-2
86. f(x) = x3(x-5)2