Definition of Relative Extrema :

1. If there is an open interval containing  on which is a maximum, then is called a relative maximum of or you can say that has a relative maximum at .

2. If there is an open interval containing on which is a minimum, then is called a relative minimum of or you can say that has a relative minimum at .

Definition of Global Extrema :

The minimum and maximum of a function on an interval are also called the absolute minimum and absolute maximum, or the global minimum and global maximum.

Now observe the graph :

The graph of the function has horizontal tangent at .

So the function has neither maximum nor at .

The graph increasing over interval .

The graph is decreasing over the interval  .

So the function has relative maximum at .

Relative maximum at .

Therefore the function has absolute maximum at .

If has a relative minimum or relative maximum at then is a critical number of .

So the function has a critical numbers at  and .

The function has a critical numbers at and .

The graph has absolute maximum at .