Let there is a function f(x).

Now differentiate it and put it 0

=> g(x) = 0    {we denote derivative of f(x) as g(x)}

Let c is the root of this equation

Now find double derivative of f(x).  Here we denote double derivative as h(x)

1. If h(c) < 0

  the x = c is a point of local maximum

2. If h(c) > 0

  then x = c is a point of local minimum

3. If h(c) = 0

  then we must find the triple derivative of f(x) and substitute in it c for x.