ORA-38759: Database must be mounted by only one instance and not open

i have received an error like the following while trying to enable flashback on my database.

sql> alter database flashback on;
alter database flashback on
*
ERROR at line 1:
ORA-38759: Database must be mounted by only one instance and not open.

Reason is: you cannot enable flashback feature when the database is in open stage.
sql>select status from v$instance;

STATUS
------------
OPEN

solution:
do the following:

sql>shutdown immediate;
sql> startup mount;
sql> alter database flashback on;
sql> alter database open;

But, we can disable flashback feature , when the database is in open state too.

To check whether flashback feature is enable or not:
sql> select flashback_on from v$database;

FLASHBACK_ON
------------------
NO

Oracle 10G/11G: Flashback features and example
Oracle 10G/11G: Different flashback options

Thanks,
have a nice day..................