$ cat file.txt
AA
BB
CC
DD
EE
FF
GG
HH
II
$ sed -n '/^DD/{p;n;p;n;p;n;p;n;p;}' file.txt
DD
EE
FF
GG
HH

or 

If you don't want the pattern,
$ sed -n '/^DD/{n;p;n;p;n;p;n;p;n;p;}' file.txt