To add a new line with specific text after the pattern is found using sed

Shell Programming and Scripting
1

hi guys,

im trying to add the following line in my xml file

<dbrollbacksegs <oa_var="s_db_rollback_segs">NOROLLBACK</dbrollbacksegs>

when ever i find the following line

<dbsharedpool oa_var="s_dbsharedpool_size">300000000</dbsharedpool>

I have succedded till adding a new line after the above searched pattern but not able to insert my specifc part (NOROLLBACK)

users/oracle> cat  file
    <dbsharedpool oa_var="s_dbsharedpool_size">300000000</dbsharedpool>
/users/oracle > sed 's/dbsharedpool>/&\n/g' file
         <dbsharedpool oa_var="s_dbsharedpool_size">300000000</dbsharedpool>

/users/oracle >

But my Output should be something like this

/users/oracle > sed 's/dbsharedpool>/&\n/g' file
         <dbsharedpool oa_var="s_dbsharedpool_size">300000000</dbsharedpool>
         <oa_var="s_db_rollback_segs">NOROLLBACK</dbrollbacksegs>
/users/oracle >

Pls help

2 
# cat file
<dbsharedpool oa_var="s_dbsharedpool_size">300000000</dbsharedpool>
<ABCD>
</ABCD>
<EFGH>
</EFGH>

#  sed '/<\/dbsharedpool>/ a\ <dbrollbacksegs <oa_var="s_db_rollback_segs">NOROLLBACK</dbrollbacksegs>' file
<dbsharedpool oa_var="s_dbsharedpool_size">300000000</dbsharedpool>
 <dbrollbacksegs <oa_var="s_db_rollback_segs">NOROLLBACK</dbrollbacksegs>
<ABCD>
</ABCD>
<EFGH>
</EFGH>